If we are programmed to have sex for the sake of reproduction...

[Cream of Caulk]

Male physiology is to blame, they need to empty their balls every so often. Their bodies produce sperm and need to get rid of it. A women's body is more efficient, we are born with eggs so we don't produce them and we naturally release and shed our egg lining once a month so sex for women is not necessary physiologically. Men need to have some type of male period for their sperm every couple weeks.