Because men play offense and women play defense.

Okay, there are a lot of problems with that. But you have to admit, there's something about that that rings true. At least in straight couples, the assumption is that men will initiate sex and women will mostly say no, but sometimes say yes.

I remember studying something like this in animal behavior. Most mammals approach sex in this pattern. Males seek out a variety of mating opportunities with a variety of partners. Since they can regenerate sperm rapidly, their best shot of spreading genes is to maximize the number of times they have sex.

Females, on the other hand, have a completely different calculus. Female mammals make few eggs and have long gestation periods in pregnancy. Essentially, they value quality over quantity. Since they have to invest a lot of time in pregnancy, they want to make sure they don't waste their time on an unfit mate.

Fast forward to 2018. When a moderately attractive woman asks a man for sex, the man loves it. He doesn't have to work to prove he's good enough because she's said yes already. His only investment is a bit of time and a bit of semen.

When a moderately attractive man asks a woman for sex, a couple things are going on. One, she probably doesn't know much about this guy and that makes her uncomfortable. Two, typically, there's some chance that she could get pregnant. Of course, there's contraception but it's not 100%. If this man is horrible, that's a huge cost with little benefit for her. Three, this man could be abusive. Is this man going to hit her? If she doesn't know or isn't at least confident of her safety, that's a problem.