I'm fully aware that a lot of people on this sub already know of this because it's widely talked about here, the purpose of this post is mainly to clarify my own thoughts on the issue and perhaps give myself and other people something to rely on in debates. Feel free to correct me if my wordings or calculations are off in any way (I easily get overwhelmed and don't handle lots of information well), because I want to produce a reliable and accurate picture of what is really going on.

Feminist Mary Koss, considered an expert on sexual violence, has pushed the idea that the vast majority of male victims of female perpetrators should not be considered as rape victims.

Here is a quote from a paper she wrote: "Detecting the Scope of Rape."

http://boysmeneducation.com/wp-content/uploads/2014/01/Koss-1993-Detecting-the-Scope-of-Rape-a-review-of-prevalence-research-methods-see-p.-206-last-paragraph.pdf

Although consideration of male victims is within the scope of the legal statutes, it is important to restrict the term rape to instances where male victims were penetrated by offenders. It is inappropriate to consider as a rape victim a man who engages in unwanted sexual intercourse with a woman.

Essentially, she has adjusted the definition to specifically mean forced penetration of a victim and excluding victims who were forced to penetrate a penetrator. This by definition excludes the vast majority of male victims of female perpetrators.

More shockingly here is an interview with Mary Koss on a radio program about men raped by women. I would encourage you to listen to the entire thing but this is a transcript of what she said:

https://soundcloud.com/889-wers/male-rape

The reporter Theresa Phung: "Dr. Koss says one of the main reasons the definition does not include men being forced to penetrate women is because of emotional trauma, or lack thereof."

Dr. Koss: "How do they react to rape. If you look at this group of men who identify themselves as rape victims raped by women you'll find that their shame is not similar to women, their level of injury is not similar to women and their penetration experience is not similar to what women are reporting."

Theresa Phung: "But for men like Charlie this isn't true. It's been eight years since he got off that couch and out of that apartment. But he says he never forgets."

Later on:

Theresa Phung: "For the men who are traumatized by their experiences because they were forced against their will to vaginally penetrate a woman.."

Dr. Koss: "How would that happen...how would that happen by force or threat of force or when the victim is unable to consent? How does that happen?"

Theresa Phung: "So I am actually speaking to someone right now. his story is that he was drugged, he was unconscious and when he awoke a woman was on top of him with his penis inserted inside her vagina, and for him that was traumatizing."

Dr. Koss: "Yeah."

Theresa Phung: "If he was drugged what would that be called?"

Dr. Koss: "What would I call it? I would call it 'unwanted contact'."

Theresa Phung: "Just 'unwanted contact' period?"

Dr. Koss: "Yeah."

The CDC has adopted Mary Koss's definition of rape in their National Intimate Partner and Sexual Violence surveys (NISVS). These surveys are already well known on this sub and need no introduction. They're the surveys that report conclusions such as these:

NISVS 2010:

Nearly 1 in 5 women (18.3%) and 1 in 71 men (1.4%) in the United States have been raped at some time in their lives, including completed forced penetration, attempted forced penetration, or alcohol/drug facilitated completed penetration.

NISVS 2011:

In the United States, an estimated 19.3% of women and 1.7% of men have been raped during their lifetimes; an estimated 1.6% of women reported that they were raped in the 12 months preceding the survey. The case count for men reporting rape in the preceding 12 months was too small to produce a statistically reliable prevalence estimate.

As mentioned earlier, the rape numbers in these studies do not cover all victims of nonconsensual sex, they only cover victims who were forcibly penetrated by a perpetrator. If a man was forced to penetrate a perpetrator, this would not be considered as rape and it would instead fall into a separate category "made to penetrate".

https://www.cdc.gov/violenceprevention/pdf/nisvs_report2010-a.pdf

Rape is defined as any completed or attempted unwanted vaginal (for women), oral, or anal penetration through the use of physical force (such as being pinned or held down, or by the use of violence) or threats to physically harm and includes times when the victim was drunk, high, drugged, or passed out and unable to consent. Rape is separated into three types, completed forced penetration, attempted forced penetration, and completed alcohol or drug facilitated penetration.

-  Among women, rape includes vaginal, oral, or anal penetration by a male using his penis.It also includes vaginal or anal penetration by a male or female using their fingers or an object.

-  Among men, rape includes oral or anal penetration by a male using his penis. It also includes anal penetration by a male or female using their fingers or an object.

Being made to penetrate someone else includes times when the victim was made to,or there was an attempt to make them, sexually penetrate someone without the victim’s consent because the victim was physically forced (such as being pinned or held down, or by the use of violence) or threatened with physical harm, or when the victim was drunk, high, drugged, or passed out and unable to consent.

-  Among women, this behavior reflects a female being made to orally penetrate another female’s vagina or anus.

-  Among men, being made to penetrate someone else could have occurred in multiple ways: being made to vaginally penetrate a female using one’s own penis; orally penetrating a female’s vagina or anus; anally penetrating a male or female; or being made to receive oral sex from a male or female. It also includes female perpetrators attempting to force male victims to penetrate them, though it did not happen.

ESSENTIALLY: under the NISVS' methodologies a woman drugging and riding a man would not be counted as rape. The numbers for men who have been raped only refer to men who were penetrated, for example men who have been forced into anal sex or made to perform oral sex on another man. And yes, situations like these are comparatively rare.

Men being forced to penetrate a perpetrator, on the other hand, is much more common, but it is not included as rape and instead gets considered as "made to penetrate". Made to penetrate falls into the category "other sexual violence" with other vastly less severe forms of sexual violence and harassment such as "non-contact unwanted sexual experiences".

However, since the CDC gathered the data for "made to penetrate", that allows us to make comparisons. One-year prevalence is considered to be more accurate than lifetime rates because they do not depend on recall of events long past, and therefore we will be using these numbers to analyse the 2010, 2011, and 2012 NISVS studies. In each of these years, when asked about experiences in the last 12 months, men reported being “made to penetrate” -either by physical force or due to intoxication - at virtually the same rates as women reported rape.

NISVS 2010 showed that in the past 12 months, 1.1% of men (or an estimated 1,267,000 men) were made to penetrate and 1.1% of women (or an estimated 1,270,000 women) were raped.

NISVS 2011 showed that in the past 12 months, 1.7% of men (or an estimated 1,921,000 men) were made to penetrate and 1.6% of women (or an estimated 1,929,000 women) were raped.

NISVS 2012 showed that in the past 12 months, 1.7% of men (or an estimated 1,957,000 men) were made to penetrate and 1.0% of women (or an estimated 1,217,000 women) were raped.

In each of the years the case count for male rape victims and female victims of made-to-penetrate were too small to provide a statistically reliable prevalence estimate.

Here is a table (source) to easily demonstrate it.

CDC NISVS Yearly Rape and Made-To-Penetrate Victimisation

Rape (women) Rape (men) Made to penetrate (women) Made to penetrate (men)
2010 1.1% (est 1,270,000) * * 1.1% (est 1,267,000)
2011 1.6% (est 1,929,000) * * 1.7% (est 1,921,000)
2012 1.0% (est 1,217,000) * * 1.7% (est 1,957,000)

\Zero or statistically insignificant amount according to NISVS)

You can see that the estimated numbers of male victims of made to penetrate each year look very similar to the estimated numbers of female victims of rape. So if made to penetrate happens about as often as rape each year then by most people's assumed definition of rape (nonconsensual sex) then men are approximately half of rape victims each year.

In 2010:

1,267,000/2,537,000 = 49.9% of victims of nonconsensual sex in 2010 were men.

In 2011:

1,921,000/3,850,000 = 49.9% of victims of nonconsensual sex in 2011 were men.

In 2012:

1,957,000/3,174,000 = 61.7% of victims of nonconsensual sex in 2012 were men.

Additionally, in 2010, NISVS found that:

For female rape victims, 98.1% reported only male perpetrators. Additionally, 92.5% of female victims of sexual violence other than rape reported only male perpetrators. For male victims, the sex of the perpetrator varied by the type of sexual violence experienced. The majority of male rape victims (93.3%) reported only male perpetrators. For three of the other forms of sexual violence, a majority of male victims reported only female perpetrators: being made to penetrate (79.2%), sexual coercion (83.6%), and unwanted sexual contact (53.1%). For non-contact unwanted sexual experiences, approximately half of male victims (49.0%) reported only male perpetrators and more than one-third (37.7%) reported only female perpetrators (data not shown).

In 2011, NISVS found that:

For female rape victims, an estimated 99.0% had only male perpetrators. In addition, an estimated 94.7% of female victims of sexual violence other than rape had only male perpetrators. For male victims, the sex of the perpetrator varied by the type of sexual violence experienced. The majority of male rape victims (an estimated 79.3%) had only male perpetrators. For three of the other forms of sexual violence, a majority of male victims had only female perpetrators: being made to penetrate (an estimated 82.6%), sexual coercion (an estimated 80.0%),

So let's examine the year 2011 here.

82.6% of 1,921,000 men made to penetrate in 2011 had only female perpetrators.

99.0% of 1,929,000 women raped in 2011 had only male perpetrators.

(The amount of women made to penetrate and men raped in 2011 were too few for a reliable estimate.)

(0.826 x 0.499) + (0.01 x 0.501) = approximately 41.7% of perpetrators of nonconsensual sex in 2011 were women.

So in 2011, 49.9% of victims of nonconsensual sex were male, and 41.7% of perpetrators of nonconsensual sex in 2011 were women. The numbers are looking more and more gender-symmetrical now.

If they didn't define "made to penetrate" as separate from rape, the CDC could have focused on a truly surprising finding - that men are half of rape victims and that women are a large amount of the perpetrators of rape, but instead they defined rape solely as "forced penetration" a la Mary Koss therefore allowing them to report such gendered numbers.