Do relationships like this actually exist? (details inside)?

I met this guy a couple months ago in my new college class. For the first few weeks we were just friendly, but in the last few weeks we've gotten a lot closer. We got more flirty/playful with eachother. We're both in our early 20's and I guess we both knew that something more than just being friends would happen eventually.

We never took it any further than friends, because I know he has a girlfriend he lives with and has been seeing several years (something that would stop me from persuing a guy). Last night he came over to hang out (like usual) and we had some drinks and were watching movies and just goofing off like normal, and we were cuddling, and he asked if he could kiss me. We were a little drunk. I said no because I knew he had a girlfriend, and asked him about it. His response was

that even though they've been dating for a few years and live together, they're not actually exclusive. They both date other people regularly and know about eachother's other relationships, but know none of the details. They'd just be considered "bf/gf" but without being exclusive.

I've never actually met this girl. I have been in the situation before where I've dated guys and later found out they had a gf they were cheating on with me, and of course I felt bad.

We did end up kissing and he swore it was ok. He stayed over (since we were drinking) and he was over for 24 hours without talking to his girlfriend even once.. he said they don't keep track of where eachother are or ask when eachother don't come home at night.

I know this doesn't sound normal, but do relationships like this actually exist? I have been friends with this guy for some time so i really don't think he's just using me, but I really don't know if he'd be the type to lie and cheat or not (hard to tell).

What do you think? Should I just trust him that it's ok or refuse just because it's possible he might be lying?