When are you considered crossing the line?

Ok, I think I f'd up! The ex is being bitter and mean, treating me horribly, checking women out in from of me, while I am doing him a favor. Well....his buddy came into town, and....well 4 years in the making came true last night! It was great, but wait..his best friend, how could I? Actually..how could he of mistreated me for all these years, everyone saw it, including the best friend! I must admit I always said to myself if he and I were not together that the friend and I would be great together, well I guess he thought so too. BUT I still feel bad, but the ex is doing his own thing, and if the friend was such a good friend to him he would not be in my bed and waiting for me to come home.
Still I question myself, but the attention, the affection and the love making is worth any consquence.