You can kill two stones for the price of one, I think this is more Odin's department though (not sure) so I can't really advise. However, since the girl you're with is seemingly obsessed with you, play the jealousy card. You can allude to the fact that this other, hotter chick wants your dick. She'll work harder to keep you:
blowjobs, sex, lude public acts, seriously..But make sure she knows that you're still open to others because you're not sure she's what you want. (more jealousy)

Then, since the other doesn't even want a relationship, just fuck her. 2 birds with one stone.

By the way, NEVER mention a relationship or anything alluding to such with fuck buddies. It ruins the whole shit. OR!!!! YOU COULD HAVE A FUCKING THREE-SOME OH MY FUCKING THOR!!! ITS AWESOME

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