made out with my roomates girlfriend

Always have kinda flirted with her, but never with intentions to do this. Both of us got drunk last night while roomate is on vacation. One thing led to another and we basically did every dispicable act in the book except having sex. We even went at it again the next morning. Not sure how I feel about this yet. It didn't feel wrong at all at the time it happened... the more I think about it though the more I realize how fucked up it is. I don't know what to think anymore.

EDIT I have decided I'm going to tell him tonight. Thank you all for your responses, this was a wake up call for me. I have been good friends with my roomate up until the past 3 months or so we've drifted apart for reasons I would have to write a novel to explain. This will not be easy, and probably friendship-ending, but he deserves this much at least. Living situation might be awkward though. Fuck.