I think the problem is that men and women think of sex differently. I'm going to generalise massively here for a minute, I'm talking about majorities, not everyone.
Most women prefer sex to be in the safety of a relationship. They want a romantic connection with their partner, they want to know that that partner isn't going to be gone tomorrow. This is logical, women bear the lifelong risks of having to raise a child in a way that men don't. Plus women are putting themselves in a much more vulnerable position by having sex with a man, men are stronger etc.
Men largely don't have this issue. We're not at risk, and as a result sex is more separate from love, from romance than it is for women. I don't need to love someone to have sex with them. I don't need to be in a relationship with them, or fancy them, or even be attracted to them, to enjoy sex with them.
Sex for men is like table tennis. Yeah, I can put one side of the table up and play by myself but it's not the same. For a really fun game of table tennis I need a partner. And I'd rather that partner be someone I get on with. The game itself is good with anyone who's fairly evenly matched, but I'm far more likely to enjoy myself if I can have a laugh with the other person. And if I'm having a game with the person I love, then I'm going to really enjoy my game, because I really enjoy everything I do with them.
I'm not single, haven't been for a very long time. But if I was, I can think of lots of friends I'd enjoy sleeping with. (Both male and female, I'm bisexual)
It sounds like a win/win. We know we have fun together already, why wouldn't that translate into the bedroom? Maybe it'll be a one time thing, maybe we'd have such a good time it'd turn into something more. But none of that means that I'm friends with these people in the hope that I can swoop in and have sex with them if the opportunity arose. I'm friends with them because I enjoy their company, not because I'm eyeing them up as a challenge round the ping pong table.
For the record, I've slept with three friends over the years. The first was male, and we went down the pub the next week and nothing had changed. The second was a woman, and things were rather weird for a good year or two afterwards.
And the third? She's still my DP 20 years later.