I asked two of my closest friends to be my BMs. One of them is in a long term relationship and he is the definition of anti-social amongst other things..
When she told him that she was going to stand up with me, he was upset about it and said things like, "why would she ask you, doesn't she have other friends?" and "I didn't know you two were so close". Which is ridiculous because we worked together for 4 years and we spoke every day - he didn't like that much either and mostly ignored the fact.
He also told her that he wasn't going to be involved - as in attend as her plus one or even babysit her kid. I don't care whether he attends or not really. I told her up front that I still want her to be my BM but I didn't want him to ruin things at the last minute by upsetting her enough that she cancels at the last minute or puts her in a bad mood on the day of. She said she was excited and she wanted to stand up with me, but she also didn't want him to get in the way of my wedding.
It's difficult for me because we've been good friends and stuck with each other through hard times. All of us are supposed to go dress shopping on Saturday but.. now I'm not sure what the best move is.