My boyfriend keeps asking for a threesome

My boyfriend of two years (26) keeps telling me how much he wants a three-some and he keeps on asking often in the hopes that my answer will change, even though I've told him many times that it's not something I'm interested in or open to. It makes me feel inadequate. How can I make him stop and give up on the idea? Also, if I don't agree to a three-some, is he more likely to cheat?