With the whole "friends" situation, I'm not sure how it works with homosexual relationships, but I can tell you how it works in a male-female relationship.
It all depends on the circumstances:
1) If a guy asks out a girl, and the girl says "I don't see you quite that way" or "I just want to be friends" she's trying not to hurt his feelings and she doesn't feel any attraction toward him. The same goes for a guy telling a girl this
2) If a guy and a girl have been dating for a while and she gives him the "I think we should just be friends" line, she's easing her own feelings. If the guy should tell her, "No, it's finished", then she'll get emotionally flustered and begin reconsidering everything.
3) If a guy and a girl have been dating for a while and HE tells her "I think we should just be friends", he's trying not to hurt her feelings (too much).
4) If a guy and a girl have been separated for a few years, it is possible for them to become friends if all the "feelings" for each other have died down.
Now, as for your situation Scooby, I had a friend (of 19 years) who treated me like your ex treated you. He was an asshole, treated me like garbage, screwed me around on the rent payments, and used sabotage to get me to do what he wanted. I've kicked him out of my life. Within the past 3 years, he's tried to "fix" our friendship by contacting me. Before I agreed to do so, I sat and thought about all the garbage he put me through. He tried to ruin my relationship with my wife, he posted a picture of us on the internet (on a site we didn't want to be on), screwed me financially, blackmailed me, insulted me, belittled me, the list goes on and on.
I set his phone number ablaze and decided to never look back.
He again contacted me and tried to guilt trip me about not inviting him to my wedding. I told him to never contact me again.
You have the power to kick these controlling, manipulative, abusive people out of your life. They don't deserve the time of day for the trouble and pain they've caused.
Also, I'm currently writing a song about how this ex-friend of mine screwed me around:
You can belittle me, but now your sentimental chains are free.