I have the exact same kink! Big time. I love seeing Merlin sleep around with everyone and everything, as long as Arthur is his OTP at the end.

And I don't think that would be necessarily unfair to Gwaine. I think he would probably be just fine with being Merlin's good buddy with benefits, either while or before Merlin is with Arthur. Gwaine does not seem at all like he's looking to settle down into a long-term romantic relationship, so a casual sex-and-mutual-adoration relationship with Merlin would be a pretty awesome situation for him.

Because Merlin deserves to get laid. A lot.