He's technically not my boyfriend but.......?

I've been dating someone for almost a yr and a half. So we're technically not together. We are sexually active w/each other and have been since 2/2010. He practically lives w/me and we act like we're together and all but like I said, we're not. I just got a phone call ystrdy from my friend tellin me that since 4/2011 he's has been sleepin w/her cousin. I'm not so much upset about the other girl or the sex part. I just feel disrespected because I opened up my home to him and helped him when he had NOTHING. Like literally. I'm going to confront him about it today, how should I in a calm matter because trust, I want to ssssnap! What questions should I ask and what should I do period?