I'm going to be referencing this post.

Let’s talk about this weird trpy obsession with a number.

Okay, so first off, it's not some "weird TRPy obsession" for a man to care about a woman's sexual history. Almost any man that was raised in a somewhat conservative or even neutral environment with two parents will probably grow up to value things like a woman who is sexually modest and ideally a virgin (especially if he plans on getting into an LTR or marriage with said woman). In fact, what often pushes many ordinary men to become Redpilled is becoming aware of just how promiscuous women are today. The majority of women are easily losing their virginities in early high school, if not middle school. Simply because you were raised in some very sexually liberal and sex-positive environment doesn't mean everyone else was either.

I'm going to partition my argument into four main components: Evolutionary Psychological and Biological Implications, Social [Status] Implications, Historical/Cultural Implications, and Female Solipsism.

Evolutionary Psychology and Biological Implications

Now I'm pretty sure the reasoning behind this is pretty obvious to everyone here as to why men. Men have evolved to not desire promiscuous women because this would be a terrible evolutionary and paternal investment on their part. With promiscuous women, there is a higher perceived likelihood that a man will potentially get biologically cuckolded and waste precious resources and effort unwittingly raising another man's offspring, which is INHERENTLY evolutionary disadvantageous.

There are other aspects in regards to Evolutionary Psychology as well. Male Primates show a high-degree of mate guarding and are very defensive about being the only one that a female primate will copulate with. It's not because these male primates are "insecure" as these feminists like to label ANY sort of male sexual preference; it's literally ingrained within our DNA and inherent, sub-conscious nature to be concerned with a woman's sexual history/promiscuity.

There is also inconclusive research in regards to microchimerism. I have a good feeling the science behind this won't receive proper funding because it's so "offensive".

Another point to look at that ties in well to female solipsism is also preselection. These women are merely projecting their desire to be with a sexually promiscuous male, because a sexually promiscuous male is deemed attractive due to how many other women find him attractive, which is explicitly related to preselection. They are confused and bewildered when they don't understand that men don't hold the same sexual attraction characteristics such as preselection and merely just assume it must be some "great insecurity" of "fragile masculinity".

Section tl;dr: Getting involved with a promiscuous woman is evolutionary disadvantageous and a poor paternal investment due to the likelihood of unwittingly raising another man's offspring. This, in turn, has evolved inherently within male nature to desire sexually modest women for raising offspring in which long-term paternal investment is required.

Women don't understand that preselection is just a female-to-male attraction principle (pareto principle describes this well as 20 percent of the men get 80 percent of the women), but this does not work in reverse.

Social Implications

How many self-respecting men do you honestly think want to be known as the guy who wifed up the town whore? I mean seriously, the mental gymnastics and philosophical relativism of these people are insane to some degree. There are also other issues in regards to this as well. Do these people really think men grow up wanting the mother of their intended children to be a whore as well?

It's well-cited of a direct correlation in marriage dissatisfaction with the greater number of prior sexual partners for a woman and that the findings are statistically significant with other deficiencies as well.

Graph: Number of Premarital Sexual Partners and Marital Satisfaction

Exhibit B, contrasting timelines, archive version

It's no wonder that they're trying to normalize and promote cuckoldry to hopefully make it more socially acceptable.

Look at how much of a difference a "mere" 3 partners makes when compared to 0, but CLEARLY, a woman's sexual history doesn't matter! /s

Section tl;dr: There is statistically robust evidence that the lower the amount of premarital partners for women, the higher the marital satisfaction (and less likely the risk of divorce).

Historical & Cultural Implications

In my honest opinion, this has to be the most ground-breaking component of all outside of the Evolutionary Biological implications.

Why is it that nearly all cultures both ancient or historical since time immemorial prioritized virginity and youth of women in terms of marriage?

As Colttaine likes to say, Culture is merely a manifestation or reflection of Biology. If virginity/sexual modesty never did matter, why was it such an unequivocal commonality among almost all cultures regardless of race/ethnicity for women to be as such? Clearly, it wasn't such a little "insecurity" of a minority of men if we saw it occur so prominently by and large throughout almost all cultures.

Female Solipsism

Now, this part of my argument has less to do with explaining why men care about how many men a woman has slept with before him and more to respond to the women in the thread talking about just how 'insecure' these 'incels' are.

It's just like what happened with the female fat shaming mantra.

Instead of actually... you know, just stop eating so much; these women go on to make PR campaigns and non-profits shaming men for not being attracted to landwhales and deluding younger women into thinking that THEY ARE BEAUTIFUL regardless of how obese they are. 'How dare men not be attracted to fat women? That's like so misogynistic! Plus size models are beautiful and THICC!'

The same logic applies to these women on Reddit. Instead of just recognizing that men have certain social and sexual preferences (and that these very women don't fit into said ideal), they somehow go through the rationalization hamstering that, "Oh, they're just insecure incels! How DARE my boyfriend ask me how many men I've been with?!?! It was only like 50 though,tots not that high!"

Just look at what happens when a man calmly disagrees with their feminist overlord.

It's no wonder that they have resorted to literally promoting cuckoldry, Exhibit B.

They are inherently solipsistic and will always vindicate themselves of any deficiency and somehow rationalize it into being men's fault.