boyfriends ex girlfriend

Ok so it's my boyfriends good friends 21st birthday coming up soon and his 2 ex girlfriends will be there. He dated them both for over a year and they're all in a close friend group so every time we go out with them they're there.
Last time one of his ex girlfriends was 'slutty' dancing in front of him on a party bus and tbh it didn't really bother me but I feel like having 2 of his ex girlfriends in a small room will make me feel uncomfortable. should I still go or should I tell him I don't want to? I'm not sure if it's harsh to say I'm not going...