Yes, and yes.

Yes, and yes.

Explanation: Men and women can indeed be platonic friends. That doesn't mean that the thought of sex won't surface in one mind or another, or that an awareness of the possibility won't result in them maintaining careful barriers. So to that extent, "sex enters into it". (It hopefully matters less if the guy is in a relationship. It definitely enters into it less if the guy happens to be gay...) I've been in this situation.

As for the romantic relationship ending and winding up as friends, I know it is possible because I know several cases where it happened. It has never happened to me, but I have multiple siblings who wound up being good friends with ex-spouses!

But I wouldn't count on either happening in any particular case. If it does, well and good. But normally a broken romantic relationship turns ugly. And frequently women who use the phrase "platonic friends" are describing relationships where she decided to be friends and he would like to be something more. Pretending that you're platonic when the other person is longing for something else just drags out the misery - the kindest thing IMO in this situation is to exit the friendship.

Cheers,
Ben