Well, that's the problem in this world all of these unwritten rules and bullshit. It wouldn't bother me. I think sometimes people are to worried about what others think, and yet your so called friends will stab you in the back the first chance they get. I say look after yourself first, they do.
You state that your "so called friends" would stab you in the back, then state that should look after yourself first. I'm assuming that means that you would stab them in the back without remorse as well. Real friends don't do that, so I would say your "friendships" are NOT friendships. Considering "unwritten rules", aka "a conscience" , to be bullshit shows little caring for others in your life. No, a friend shouldn't dictate your life, they should be accepting. The reverse is true, and if you know that something is going to hurt someone you supposedly care about, and that doesn't matter to you? Basically, it means you don't really give a sh*t about that person, period.
I have friends from my childhood who have been there for me thru thick and thin. Thru relationships and hardships. Thru my husband's death and my LTR and I ending. They have understood when the relationship has taken priority, as is healthy, and have always been supportive and honest.
Would it bother me if a friend dated an ex? It really depends on the ex and the type of relationship I still had with them. If it was someone that they really liked, my wish would be for them to be happy.
However, they would never do so without first discussing it with me. Whether it was just for sex or for more, they would tell me first.
That you have a concern, MCA, means that you believe there may be an issue. Your hesitancy shows that it would bother you if the situation was reversed. How would you feel if a friend did it to you, and never discussed it with you first? If he's really your friend, you will approach him with it.