How do I tell my boyfriend?

So, I’ve been with my boyfriend for almost a couple years...and there’s something I’ve never told him and kinda have always wanted to but don’t know if it’s a good idea.

I want to tell him that I’m bisexual but don’t know if I should. Not that I like keeping it from him but I’m not sure how he’d react exactly. Part of me thinks he’ll understand, but another part thinks he might not want to trust me because the whole stigma against bisexuals (🙄), or that he might like it but because he thinks he can get threesomes out of it..

Super conflicted...what should I do?