falling in love?

so, i'm a 22-year-old guy living in seoul (moved there from busan). i've never fallen in love - i make it a rule never to sleep with the same person more than once (i'm more of the one-night-stand type), and i'm pretty content that way. relationships are too messy, and i have no interest in them.
however, there's something that's been irritating me lately. i slept with this guy recently, and actually started talking to him (over katalk) (we'd also go out for drinks). i didn't have a relationship in mind - we just talked about shit like our latest "conquests" (or so we called them - as a joke, of course). i later learned that he had a girlfriend (dude, why are you even sleeping with other people when you have a girlfriend?), and that he was "just experimenting" with guys. why did that make me feel slightly disappointed? i'm not sure what i was expecting from a kinda-friendship (that had once been only a one-night-stand), and i'm seriously pissed off at myself for having these weird thoughts. i'd vowed never to fall in love, and i was sure i had pretty decent control over my emotions - what the fuck are these thoughts? he's even got a girlfriend (straight guy, probably, who had one too many drinks and decided to do it with a guy). but, get this - the other day, the asshole started rambling about having feelings for me and shit (after he had just a few drinks), and tried to kiss me. my brain was so fucked up at the moment that i just paid the bill and left without saying anything (a dick move, but i was also pretty drunk, so it's not as if i was thinking straight). i'm pretty sure he doesn't even remember what happened yesterday, as he's been messaging me as per fucking usual, as if nothing happened. the fact that i'm having these feelings pisses me off, and the fact that they're for a dickhead who's screwing around behind his girlfriend's back makes me even more irritated. so - advice?? i don't even fucking know.