NTA And I don’t think I could stay with a person who A. Doesn’t respect my stated boundaries and kept bringing this shit up, and B. Tried to make me think my very valid feelings were “irrational” simply because they don’t feel the same way, and in an effort to purely get what they want.

OP just because you’re bisexual doesn’t mean you are required to have a threesome. And nobody is irrational for not wanting to have one for any reason. Usually in a threesome the whole point is for all 3 people to be comfortable and enjoy the experience. Him knowing how you feel and trying to force it on you tells me he just wants to fuck someone else and needs you in the room so it’s not technically cheating.