Must the "BestMan" be single?

Me and Beary have been debating for quite some time on his 'choice' of bestman. I told him that following the chinese culture, it's better that he chooses a single guy to be his bestman, as bridesmaids need to be single too (according to my mumzy).

And today, he still insists on taking his married friend to be his bestman, as he was the bestman for that friend previously... So it's like an obligation... Out of his 5 close friends where 4/5 are SINGLE, he has to choose the married one!

Sometimes I don't even know why I bother debating with him coz we're both looking at it in a different angle

Anyone has anything to say about this culture?