Is this cheating?

So my boyfriend has been wanting to explore our relationship with a 3 or 4 way when we’re drinking sometime. I’m not completely opposed to the idea I think it just really depends on the person or people for me to want to do it. Lastnight we were drinking with our roommate and his girlfriend that live in our basement and the 4 way came up. I didn’t want to due to who they are and “went to bed” to avoid talking about it any more . Me and my boyfriend have been together 4 years and never had any problems with trust but while I was “asleep” as they thought, I heard a conversation. Our roommate said to his girlfriend “just show him something he hasn’t seen anything of yours, just show him your tits or something so he can get himself off later” and she said “mmm idk maybe next time” and then they went downstairs. I confronted my boyfriend about it and he denied it at first and then said well nothing happened over it so I don’t know why you’re so upset. I was like ?!????? Why the f*** didn’t you say something???? Like am I crazy for being mad about this. we are on the verge of breaking up because I am so upset. I don’t really know what to think or do.