It’s not that platonic friendships can’t exit, but they are usually a very delicate balance where one person is getting some kind of romantic relationship validation. This isn’t usually needed when a full, sexual relationship comes along, like your friend becoming more distant.
My male friends are using me for this validation and me them at the moment, I could be very naive and say that there isn’t any attraction, but of course there is. They make me feel wanted, available. I would be more distant but not drop them completely if I got into a full relationship again. One of them has a girlfriend now so we now just naturally only meet up in a group, the coffee, theatre meet ups one to one have stopped, as they should.
Then your idea of male-female friendship is very different to mine. My male friends don't make me feel 'wanted' or 'available', any more than my close female friends do -- whereas you seem to see your friendships with men as some kind of placeholder that involves mutual sexual validation until one of you gets a boyfriend/girlfriend, and then it turns more distant because you're getting that validation from someone else.
I'm not being 'naive' when I say that my male friendships don't operate this way. We're all longterm happily married, so these friendships aren't some kind of practice run for a 'full sexual relationship' we've all already got one of those. But marriage isn't, and it shouldn't be IMO, exclusive in the sense that you are not close to other people both DH and I need more relationships in our lives than the one we have with one another, and they make our marriage happier.