I'm not going to allow my mother's boyfriend at my wedding
Why are you choosing to ban your mother's boyfriend from your wedding?
It's because of the latter. He used to be some sort of activist, but nowadays he has become an obnoxious self-absorbed jerk.
It's your wedding so you should be able to choose who can go to your wedding. Even your own parents are not exempt since this is your wedding after all.
That's quite condescending. I think I have the right to bring who I want to my wedding. Maybe I don't want anything to do with anyone at my wedding.
This is your event and if you feel as though having your mother's boyfriend there will make it bad for you, then you don't have to invite him.