I didn't know he had a girlfriend - I feel like a slut...was it my fault?

Last night I was out at a club and a guy kept looking at me and he was cute so I thought I'd ask him if he'd like to dance (it did cross my mind that we might end up making out but for starters I thought I'd just see if he wants to dance with me). We were kinda grinding and his hands were wandering everywhere. We were dancing right next to where his friends were and he didn't kiss me but asked me if I wanted a drink and led me off the dance floor. After he got drinks he started leading me in the opposite direction from where his group of friends was...until one of the guys in the group came up and pulled him away from me, before going to me "that's my sister's boyfriend!" as though I was a slut for hitting on a guy who wasn't single.

I think the guy was planning on taking me somewhere his friends couldn't see him to make out with me, thereby cheating on his girlfriend (although just to clarify, we never ended up kissing). But I honestly had no idea and wouldn't have even bothered asking him if he wanted to dance with me if I'd known. I felt so ashamed and embarrassed! They probably thought I was such a slut and I bet his girlfriend's brother was thinking some very nasty things of me for it. I feel so horrible, but was that my fault or should he have told me right from the start that he had a girlfriend and turned me down when I asked if he wanted to dance?