Dealing with a threesome?

My bf wants a threesome. I just can't seem to get past the idea that if we were to have a threesome (2 girls) that it's because he wants her. How do you wrap your mind around that? How do you keep from letting your thoughts drift there? I just can't help but wonder that I should be enough but this might be his way of getting more without actually cheating? And it's not a problem to say no to him, by the way. That's not the problem here. It's the fact that he wants it that gets to me. Like I already heard he wants it and now I can't help but see him differently. Not as in love and crazy about me as he's always been. Please help!