Over any given 12 month period, it's rare for there to be more then a 10% difference between the two. There are still more male rapists due to higher rates of male-on-male rape compared to female-on-female rape, but neither stat is very high in absolute terms. Contrary to what you've probably heard in the past, most men are not raped by other men. ~80% are raped by women. And more men in total are raped every year than women, making sexual assault more of a men's issue than a women's issue.

Lifetime stats follow this as well, but not as closely. Those stats include all forms of sexual abuse, including things like flashing and groping, and they are less accurate than yearly stats for a number of reasons. The largest being because men, over time, will reframe a rape as consensual due to gender norms (everyone telling him he was "lucky", like if a teacher did it)... speculation is that women may do the opposite, thus moving their stat in the opposite direction. But even the official, unfiltered lifetime stat isn't that far off: 1 in 4 for women and 1 in 6 for men.

Why is this not very well known?

Several decades ago, an early gender studies "researcher" named Mary Koss proposed that a person "forced to penetrate" another person is not in fact raped. She called it unwanted sexual contact and she was very successful in convincing academics and even lawmakers to adopt this definition. While technically it is gender neutral, due to the anatomical differences between men and women, it mainly applied to men. Her rationale was that men must "want it" or are "asking for it" so it doesn't count.

As a result, the true rate of sexual assault against men is usually hidden in official statistics. Instead of rape, they call it "made to penetrate". And many state and national laws around the world similarly define rape in gendered terms, meaning female rapists avoid prosecution and generally don't get discussed in the media. This has led to decades of misinformation on the topic. And it has only just recently started getting attention, both on the legal front, and among academics.

Unpopular? Definitely.

Factual? Absolutely.

Inconvenient? Maybe if you're sexist. That is in fact likely the motivation behind people who try to deny it (I'm looking at you, Mary Koss).

Sources: A good meta-study

Stemple, L., Flores, A., & Meyer, I. H. (2017). Sexual victimization perpetrated by women: Federal data reveal surprising prevalence. Aggression and Violent Behavior, 34, 302-311. Available from: http://webshare.law.ucla.edu/Faculty/bibs/stemple/Stemple-SexualVictimizationPerpetratedFinal.pdf

Some pop science articles

Stemple, L., & Meyer, I. H. (2017). Sexual victimization by women is more common than previously known. Scientific American. Available from: https://www.scientificamerican.com/article/sexual-victimization-by-women-is-more-common-than-previously-known/

Young, Cathy. (2014, September 17). The CDC's Rape Numbers Are Misleading. Time. Available online at: https://time.com/3393442/cdc-rape-numbers/

Rosin, Hanna. (2014, Arpil 29). When Men Are Raped: A new study reveals that men are often the victims of sexual assault, and women are often the perpetrators. SLATE. Available from: https://slate.com/human-interest/2014/04/male-rape-in-america-a-new-study-reveals-that-men-are-sexually-assaulted-almost-as-often-as-women.html

43.6% to 48% of self-reported perpetrators are women

Ybarra, M. L., & Mitchell, K. J. (2013). Prevalence rates of male and female sexual violence perpetrators in a national sample of adolescents. JAMA Pediatrics, 167(12), 1125–1134. "2012 study using data from the U. S. Census Bureau’s nationally representative National Epidemiologic Survey on Alcohol and Related Conditions". Quoted in "The Understudied Female Sexual Predator". Available here: https://www.theatlantic.com/science/archive/2016/11/the-understudied-female-sexual-predator/503492/

Average past 12 month rate of sexual assault when "made to penetrate" is included:

US Sexual Assault Rates: Average 12 month prevalence (2017 CDC NISVS State Report). Graph: https://i.imgur.com/J5AuzM7.png In 2012, one out of three years recently analyzed by the CDC, more men were raped by women than the reverse

New CDC data again finds as many (if not more) male victims of female rapists than female victims of male rapists. CDC continues to ignore its own shocking findings. Recalculating The Gender War. https://recalculatingthegenderwar.tumblr.com/post/162336650896/new-cdc-data-again-finds-as-many-if-not-more

Men are less likely to view things like bad / unenjoyable sex, drunk sex, and capitulating to sex with a persistent person, as rape (which likely effects reporting and statistics on the topic)

Weiss, K. G. (2010). Male sexual victimization: Examining men’s experiences of rape and sexual assault. Men and Masculinities, 12(3), 275-298. Abstract: https://journals.sagepub.com/doi/abs/10.1177/1097184x08322632

Reed, R. A., Pamlanye, J. T., Truex, H. R., Murphy-Neilson, M. C., Kunaniec, K. P., Newins, A. R., & Wilson, L. C. (2019). Higher rates of unacknowledged rape among men: The role of rape myth acceptance. Psychology of Men & Masculinities. Abstract: https://psycnet.apa.org/record/2019-39470-001

Men are 9 times less likely to report their assault compared to women (which also distorts reporting and statistics on the topic)

Toy Soldiers. "Forced to Penetrate Cases: Lived Experiences of Men". https://toysoldier.wordpress.com/2020/01/05/forced-to-penetrate-cases/

Weare, S. F. (2017). Forced-to-penetrate cases: Lived experiences of men-Baseline Research Findings. The Law School, Lancaster University. PDF: https://wp.lancs.ac.uk/forced-to-penetrate-cases/files/2016/11/Project-Report-Final.pdf

Women receive a broad pass when it comes to sexual assault, especially sexual assault against young children, which distorts federal reporting and statistics on the topic

Denov, M. S. (2003). The myth of innocence: Sexual scripts and the recognition of child sexual abuse by female perpetrators. Journal of Sex Research, 40(3), 303-314. Available from: https://www.ncbi.nlm.nih.gov/pubmed/14533025

Embry, R., & Lyons Jr, P. M. (2012). Sex-based sentencing: Sentencing discrepancies between male and female sex offenders. Feminist Criminology, 7(2), 146-162. Available from: http://www.ncdsv.org/images/fc_sex-basedsentencingdiscrepanciesbetweenmaleandfemalesexoffenders_2012.pdf

"Forced-to-Penetrate" treated different from rape Weare, S. F. (2017). Forced-to-penetrate cases: Lived experiences of men-Baseline Research Findings. The Law School, Lancaster University. Available from: https://wp.lancs.ac.uk/forced-to-penetrate-cases/files/2016/11/Project-Report-Final.pdf

Weare, S. F. (2019). Experiences of men forced-to-penetrate women in the UK: Context, consequences, and engagement with the criminal justice system. The Law School, Lancaster University. Available from: http://wp.lancs.ac.uk/forced-to-penetrate-cases/

Weare, S. (2018). ‘Oh you're a guy, how could you be raped by a woman, that makes no sense’: towards a case for legally recognising and labelling ‘forced-to-penetrate’ cases as rape. International Journal of Law in Context, 14(1), 110-131. PDF: https://pdfs.semanticscholar.org/c40f/3f3dfeeaa75fed2c4ee01673f2fdfb06948f.pdf

Klee, Miles. (2018). WHY CAN’T THE MEDIA CALL A WOMAN RAPING A MAN WHAT IT IS? Mel Magazine. Available online at: https://melmagazine.com/en-us/story/why-cant-the-media-call-a-woman-raping-a-man-what-it-is

Siobhan Weare, Lancaster University. (2017). Men forced to have sex with women aren't being properly recognised by the law -- legal expert. THE CONVERSATION. Retrieved October 24, 2019, from http://theconversation.com/men-forced-to-have-sex-with-women-arent-being-properly-recognised-by-the-law-legal-expert-81638

Rumney, P. N. (2007). In defence of gender neutrality within rape. Seattle J. Soc. Just., 6, 481. Available from: http://eprints.uwe.ac.uk/14750/1/RumneySJSJ.pdf

Criticism of definitions given by the CDC

Stemple, L., & Meyer, I. H. (2014). The sexual victimization of men in America: New data challenge old assumptions. American Journal of Public Health, 104(6), e19-e26. Available from (HTML): https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4062022/#!po=0.877193

Rape laws defined in such a way that women tend to get off the hook, and not counted in government statistics

Sexual Offences Act 2003, c. 42. Available at: http://www.legislation.gov.uk/ukpga/2003/42/section/1

Many US states and many countries around the world have gendered sexual assault laws, usually defined in terms of penetration.

Not all is great in the world of men: a reference book of men's issues. Section 3.4, "Rape laws excluding male victims". https://www.reddit.com/r/rbomi/wiki/main

Male rape victims forced to pay child support Muller, R. (2019). When Male Rape Victims Are Accountable for Child Support. Psychology Today. https://www.psychologytoday.com/intl/blog/talking-about-trauma/201902/when-male-rape-victims-are-accountable-child-support

S.F. v. STATE EX REL. T.M. (1996) Summary: http://al.findacase.com/research/wfrmDocViewer.aspx/xq/fac.19961122_0042048.AL.htm/qx

ALIA BEARD RAU. (2014). "Statutory rape victim forced to pay child support". USA Today. Available at: https://www.usatoday.com/story/news/nation/2014/09/02/statutory-rape-victim-child-support/14953965/

Role of feminism Stemple, L., & Meyer, I. H. (2014). The sexual victimization of men in America: New data challenge old assumptions. American Journal of Public Health, 104(6), e19-e26. Available from (HTML): https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4062022/#!po=0.877193

Berger, R. J., Searles, P., & Neuman, W. L. (1988). The dimensions of rape reform legislation. Law & Soc'y Rev., 22, 329. Feminism: Legal Aspects. (n.d). In Law Library - American Law and Legal Information. Retrieved November 11, 2019, https://law.jrank.org/pages/1222/Feminism-Legal-Aspects-second-wave-critique-rape-law.html

TNN. (2013). Activists join chorus against gender neutral rape laws. The Times of India. Available at: https://timesofindia.com/india/Activists-join-chorus-against-gender-neutral-rape-laws/articleshow/18840879.cms

Prominent feminist Mary Koss admits to purposeful redefinition of rape to emphasize female victimization and exclude male victims

Koss, Mary P. (1993). Detectin the Scope of Rape: A Review of Prevalence Research Methods. Journal of Interpersonal Violence, Vol. 8, No. 2, 198-222. Available at: 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

Recorded interview with Mary Koss. https://soundcloud.com/889-wers/male-rape