Advice? No. Harsh truths? I've got plenty!
So, let me get this straight: you like the guy enough to break his girlfriend's heart, apparently there's some sort of shared lust (those "feelings" he supposedly has for you, which, trust me, he doesn't), but there's no actual relationship potential here since you yourself would never date him. And your excuse is "If it wasn't me, it'd be somebody else?" Really?
What you're doing is wrong, and I can tell from your question and your lame justification that you know it's wrong. So knock it off. Bad enough you've decided to hurt somebody else so you can have a human vibrator, don't compound it by trying to shove off the blame.