The only head you need to worry about is yours, because somehow, you've lodged it deep within the confines of your ass.
Let me turn your question around on you: if this woman you love admitted that she had deep feelings for you and wanted to be with you forever, and so didn't want to cheat, but there's this guy she knows with the magic tongue she just can't resist...exactly how angry and hurt would you be? And how much credence would you give to her protestations that she loves you?
Because that's exactly how angry and hurt she'll be.
Head is five minutes, love is forever. It doesn't matter if this girl can suck a ping-pong ball through a drinking straw. If you really love your lady, it's well past time to walk away.