My Best friend m34 is getting married to f29 in August. I refuse to be the best man because they have an open relationship AITAH?

I m33 know him since Kindergarten. I don’t want to testify that marriage since I am against the idea of Open relationships. I want him to be happy of course. But I don’t want to be the best man for an Open relationship.
AITAH for refusing to be the best man?