I've been thinking about this recently, too, and while I'll avoid giving you the full version of my thoughts, here's the thing: men are way hornier than women and are way more willing to sleep with random women then women are to sleep with random men. This means that the SOLE restraint that keeps a woman from sleeping with a dozen guys is her willingness to sleep around (read: it's inside her control). Conversely, a SOLE major restraint that keeps men from sleeping with a dozen women is the women's willingness to sleep with him (read: it's mostly outside his control). This puts men at a real disadvantage. She has an very easy way to really screw over his emotions and the relationship, if she wants to. Women always have the sexual advantage over men, since men simply can't find women to sleep with them as easily as women can. (I've heard some crazy stories about this kind of stuff - for example, one of my friends slept with a woman who was married because she was getting back at her husband for cheating on her. The night after sleeping with him, she slept with a second guy. That's just how easy it is for women to sleep around - in retribution, she can sleep with two different guys on consecutive nights. Guys are not typically in that position because women tend to be less willing to sleep around with some random guy.) If she has slept around a lot, it means that she's got a low restraint on sleeping around. I encountered this recently with a woman. She's polyamorous. We dated for a few weeks. One thing that became very apparent was that she could sleep with a dozen guys (that she liked) in less time than it took me to find a single girl to sleep with (that I liked). It was very disorienting and emasculating.
I also wonder if it has something to do with the shape of our sexual organs. For men, after sleeping with a woman, his genitalia is easy to wash. When a woman sleeps with a man, she can't really clean up as easily. Her genitalia is shaped like a cup that's difficult to clean. I've slept with girls who have slept around, and I don't go down on a girl if I know she's sleeping around because who the hell knows what's inside there.
There's also the issue of reputation. Because men often don't respect the women they're sleeping with - particularly if it's just a short one-time thing. If a woman has a high partner count, she probably has quite a few of these. If a guy doesn't respect a woman, and you (another guy) make her a girlfriend, it's sort of an insult to you (as a guy). It's the reverse of pre-selection. In pre-selection, you think more highly of someone if lots of opposite-sex people want him/her. Conversely, if some other guy(s) see your girl as someone who's worth a shag, but not worth making into a girlfriend, it suggests that she's not worth being a girlfriend. Which suggests that you (the guy) are kind of loser if you make the girl (who wasn't "good enough" for him) into a girlfriend. Alternatively, if they were in a long-term relationship together, I'd expect him to be much more respectful and the breakup was probably difficult, which implies a certain respect towards her. Disrespectful one-night-stands are correlated with high partner counts. LTRs aren't typically correlated with high partner counts.
It's also possible that it's related to men's fear of cuckoldry and raising children that aren't their own. Women have an advantage in that they never have to question whether or not their children are their children. I realize that "high partner count" and "cheating" are two different things, but if a girl is slow to sleep around, it means she probably is going to be slow to cheat on you sexually, as well. Afterall, if a girl makes men wait months to sleep with her and only after establishing a strong relationship, then she probably has a low partner count, and she probably isn't quick to cheat, either (she'd make the "other man" wait for months and jump through all kinds of hoops first, too, which reduces the chances that she'll end up cheating). Conversely, if a girl has a high partner count, it suggests that she'll jump in the sack quickly with a guy. While she might be loyal enough in a relationship that she wouldn't do that, it's riskier that she will. We might say that the risk of cheating sexually is inverse to (loyalty + slowness to jump in bed). "Slowness to jump in bed" is going to be correlated with a low partner count and correlated with an unwillingness to cheat sexually. In my own personal experience, it seems like women that I've slept with have a correlation between a high partner count and a willingness to cheat. (I know this because they were either cheating with me, or they told me that they had cheated on men - boyfriends and fiances - in the past.)