My boyfriend think I am lesbian for wanting to have a threesome with my girlfriend

Yesterday, I talked about having a threesome with my boyfriend. He was interested at first. We were fantasizing about it for a while. We are both swingers, so I thought we wouldn't mind who we are going to have a threesome with. We talked about how, when and where to have it. Then we moved on to who. I said to him that I have a choice. That was one of my girlfriends. She is very pretty and nice. The most important of all, she is also a swinger and illness-free. However, my boyfriend thought I am a lesbian for wanting to have sex with my girlfriend and tried to break up with me. What should I do?