Boyfriend Might Be Gay - Help

Me and my boyfriend live together in an apartment. It was a Thursday afternoon, and I had to work late. When I arrived home, I found my boyfriend of two years having sex with his best friend. Of course, I got furious, and I demanded an excuse. He didn't have one other than that they just wanted to try it out.

What the hell? Should I leave him or not? Nobody wants a gay boyfriend. Please help.