What explains the biological reasons for sex?

This is getting into deep Biological conversation and/or survival of the fittest type of stuff. But why is it that women really will only have sex if you are serious and want to have children? And men want to have just sex, but don't want children. Even if you are in a serious relationship with someone you've been with for years. Maybe I'm meeting the wrong women, but overall, across the board that is what it is. Men want to have sex just to have sex, but the girl will have sex only if you are serious and want to have kids.What explains the biological reasons for sex?We have an animal nature as humans and sexual urges are a large part of this nature. Hormone levels determine how strong those urges are. Testosterone is usually higher in men and thus stronger sexual urges. We also have minds to govern our behavior. Families are the building block of society. Women have a biological clock where they are able to have babies or have healthy pregnancies. It is because of the sexual liberation of women that women are having a hard time getting men to commit. Men do not have the biological clock to deal with and there are many women having sex without the commitment of marriage. If you are not getting with these women it is probably because you are honest about just wanting sex without commitment. Many men say and do what they need to do to get sex. This probably won't change unless the majority of women stop having sex unless they have the commitment of marriage.