Without knowing anything about you except you asking this question, I know you are a man. Even though we have evolved over time, we still have the same biological urges, and for men, it's to "spread your seed" to as many women as possible, and for us women, to seek out the strongest, most attractive male to have the strongest, most attractive offspring. So for men, they just need a reason not to want to have sex with a woman; for women, we need a reason to want sex with a man, and just because a particular man is there isn't enough of a reason. We know men are wired the way they are, and that you are in competition with other males for us, so we don't want to settle for any guy who wants to fuck us...we'll wait until we think the guy is strong and attractive enough for us. That is all deeply rooted within us, and it works against men's desires to screw every attractive female.

Women have a "made my skin crawl" reaction to guys they don't find attractive and desirable wanting to fuck them. We want a strong and attractive mate, and we know that isn't it. What can happen is the woman's natural response is either ignored or not perceived when emotional issues, drugs and alcohol, etc. affect her decision-making, and women fuck guys they wouldn't have fucked otherwise. Then, they regret it later. Studies have shown sexual regret is different by gender - men regret what they didn't do; women regret what they did do.

People are very quick to blame religion for why people don't go around screwing as many people as possible, but religion fucks with sexuality more by making those natural urges seem sinful, and by preaching that sex is only for procreating within an "appropriate" marriage.