just tell your husband that about 80% of the human population enjoys butt play weather they will admit it or not lol plus the anus has just as many nerves as the genitals.

He will figure himself out, he just needs a little time

Liking butt play does not make a man gay or even bi. The fact that he was repulsed is further evidence that he's strictly straight. Maybe ask him what he does want instead of making suggestions.

Others have already addressed the main issue, I'll address your problem. Being bisexual doesn't mean they'd be interested in threesomes or would be okay with hooking up with others while in a monogamous relationship.

Maybe a plug is as far as he wants to go?

Sounds like he wants a dead badger.

Maybe stop suggesting to bring other people into the relationship? It's not like all bi people exclusivly wants threesomes. He seems happy with just you and a buttplug, why do more?