Source? I think it's reasonable to believe that a significant percentage -- if not a majority -- of rapes and paternity fraud cases go unreported. How do you know which is more rare?

Maybe I wouldn't, but then again, neither would you or anyone else. I think most adulterous women use protection in order to avoid having this happen (mostly because they don't want to get caught), whereas rapists deliberately rape. But, of course, more people cheat than rape.

I'm asking why it happens.

Okay, we can talk about anatomy then. PiV sex provides little stimulation to the clitoris (the reason most women can only get off via foreplay) and a guy on a ONS is more likely to do a jackhammering PiV sesh than actually go down on you or do anything that involves the clit. It's not a matter of "Too many guys hit on me for me to get off on this", it's "My body is not designed to orgasm from this action."

Also, notice how you're putting the responsibility for the woman enjoying sex entirely on the man.

I'm not. No guys is obligated to do sexual things they don't want to do, but women also aren't obligated to sleep with them. I'm sure there are plenty of women who are bad in bed too, but men can orgasm even from bad sex, and women can't. I never said if she doesn't get off it's his fault. It's how her body is made. It's nobody's fault. I just know how my body is, as do many other women, and because we know it would take extensive work for an orgasm to happen with a random and strange partner, we don't bother. No knock on the men, it's just not worth it.

And gross? What do you mean by gross? If it's a woman thinking "penises are icky" then she's not really mature enough to be having sex anyway, so I don't see how that would be relevant.

Maybe this is strictly a woman thing, but I do find having a stranger I just met put his body part inside my body part to be pretty gross, and I absolutely love having sex with my husband. Many women don't feel comfortable having sex with strangers, probably due to a variety of biological impulses. That's why it's gross.