Do not complain about other subs here or post to push an agenda. Because dating an ex that your bud is still in love with is super fucked up. I think you are overlooking the fact that breaking up with someone can be just as bad as being broken up with. And that's enough for A to not want to be friends. It hurts A to hear about C's new romance with her ex, or it is awkward for C to try and not talk about it at all around A.
What a relationship starts, unless exceptional conditions, there must have been some sexual or romantic tension before that, and this happened while the previous relationship was ongoing.
I told him I was going to ask her out because I didn't want it to seem like I was going behind his back. Depends how long ago they broke up. What you're implicitly stating when you begin dating a friends ex-girlfriend is that you value your own desires, pleasure, lust, etc. Dating your friend's ex. You better keep your fucking hands off her.