I tried to remain friendly with a couple of my exes, even wrote about it here. Didn't stay that way.
Each had his own agenda, which meant trying to reenter my life at his convenience, for his own advantages.
One even tried to break up my relationship with my late partner. That doomed any future that guy might have had with me, on any level.
And the other one also didn't want to accept that I had moved on to having a partner. That he had missed the boat with me, by his own choice.
I had asked him for 2 years to be my partner, and he refused. Then I gave up and moved away, and found a partner. Not 2 weeks later he was trying to get back together again with me, said he realized his mistake, he really always loved me after all.
Well, maybe so, but your timing sucks. I already made a commitment, the one you kept rejecting for 2 years. And I don't dump a guy to whom I've just said "yes", because you've had a sudden change of heart. Nor should you expect me to stand by at the ready, for the rest of my life, doing nothing until it suits you to take me down from the shelf and use me.
You said "no" to me for years. Where was your own "yes" then? And how do I know your commitment is really sincere, not just your regret & jealousy that somebody snatched me up? I know this other guy is sincere, and I made my pledge to him, and I don't go back on my word.
So I tried to maintain a friendly relationship with him for a while, but I could see he was just playing the vulture, waiting in the wings for his chance. That was unfair to my husband, and frankly not healthy or productive for him, either. Time to move on. So I cut my ties with him.