I know that each situation is unique but isn't it better for both people to just move forward and always keep the memories of what was and leave it at that? Is it possible that there is just too much history with someone or because you knew each other intimately at one time, it would be odd to regress to a lower level such as friendship? Same can be said for a crush, sure you never got to a relationship but because you had feelings for that person, wouldn't you just be torturing yourself by keeping them in you life even as just a friend?
When I was 14 or 15, I can't remember, I met this girl when I was at the gym. She was chubby, walked in with a few of her friends. She started talking to me, impressed by what I was doing when I was actually very weak back then. She came to the gym again and talked to me. I developed a crush on her. I became friends with someone and when I went to his house, I saw her there with his sister. She started coming over a lot more. She would flirt with me and have a desire to feed me candy and stuff after she had put it in her mouth and was covered in her saliva. I was making a playlist on a friend's computer and she wanted to use it. She told me that if I didn't let her she would lick me, and she did. She didn't stop though, she was licking my neck for a good 5 minutes or so. This stuff went on for a while. She was first crush but I never went beyond any of this.
10 years later, we're friends. She gained more weight than she used to have. We still talk, sometimes about the old days. There's a part of me that I feel will always want to have sex with her, not because I'm crushing on her, but because I didn't back then. No feelings between her and I though. Just friendship.
I can be friends with a crush, yes, and I've done it several times before. I typically treat all women the same, whether I like them or not, so very few of my crushes have ever actually known that I was interested in them.
It's different when an ex is involved, especially if you've had sex with them. You can't forget that level of intimacy and connection, and keeping them around in one's life will only court relapse. That's the last thing a person needs when they're in a relationship with someone new. You're better off to just get them out of your life as much as possible.
As one musician put it: "Don't pretend that we could ever just be friends after I've been inside of you."
I think if someone had a crush, it eventually fades and doesn't matter much anymore, but an ex, well there would be history there, and it is better to just stay away from each other. They're an ex for a reason, where as a crush might just have been unavailable or something, but nobody necessarily mistreated the other to cause a break up.