Three some.?

My SO has brought up the idea of a 3-some jokingly and I said no. After a while and usually it's after we have sex he manages to bring it up again. I've explained my reasons to him and said its weird. And he says ok and goes along with but then a couple days later he asks again. Is there some type of thrill of you do this? And how do I get him to see my way on this subject?