BF already committed to going to OP's friend's wedding, long before this date was even selected for the bachelor party. Also, OP has already gone to two weddings for their BF for college friends of his that they never met.

He should have been more considerate towards his partner, who he had already made plans with months in advance, and told his friend, "Hey, sorry, man, but that date doesn't work for me - I'm already booked that day for a wedding and have been since February. Have a good bachelor's party and I'll be there in spirit. Maybe we can do lunch to celebrate it just us next week? My treat."

Also, it's bad/poor wedding etiquette and societal norms to not go to a wedding you've long been rsvp'd for for a bachelor party - the bachelor party is not the important event that can't be missed. It's his friend's actual wedding. It's also rude and shows a lack of consideration and respect for his partner to blow of their friend's wedding for a bachelor party that was not planned as in advance as a wedding, especially when OP had already gone to two weddings for people they didn't know for their partner.

BF is putting his friends above OP and commitments he already made and that's not okay.