Jealousy
For several years, I had a great bromance. He was bi and Im gay. There was nothing sexual between us and I didnt want there to be. We could talk about anything and everything. We gave each other support during hard times. We also laughed a lot. We were (and are) nudists. Over the years, he dated some truly terrific women and Im still friends with a few of them, including his ex-wife. The guys he dated were also good people.
Then he started dating a woman who was pathologically jealous of any and all of his friends. She impersonated him by sending texts from his phone. She bullied some friends, and told him lies about the others. She thinks that stalking is perfectly normal. When I or anyone else tried to tell him about her, she convinced him that we were just jealous. Everybody was trying to steal her man. I now keep Mace on my nightstand in case she finds me.
Since then, Ive completely avoided any kind of friendship with a straight or bi man. Never again.