Is a guy with a live-in girlfriend completely off limits?

I've been friends with a guy for about a year. We met through work and always look out for one another. He's great fun and we have a flirty rapport but I hold the line since he does have a live-in girlfriend and has discussed openly with me that he plans to propose to her. However, the other weekend his gf was out of town and we were at a party and he'd had quite bit to drink. Long story short, while driving him home, he was fondling my leg and while at a diner to sober up a bit he emphasized that he wasn't married, wasn't engaged and had not asked her father for permission as he had planned over Thanksgiving. Apparently, the girlfriend told him it wasn't necessary. He said he liked me and I admitted the same in kind. However, I said we have this great flirty thing and if this happens...well, we probably won't have it anymore. After more fondling my leg, we arrived at his home, he propositioned me and I told him if this were to happen it seemed to me it should be some place other than the place he shares with his girlfriend. That seemed to shock him back into reality. He said he didn't want me to hate him and decided to kiss me and say good night. Now kissing me involved some serious and surprising ...ended...continued and then he left. It was fantastic and he is the first guy that I've really liked in a long time, but I'm willing to relish the great fun it was and chalk it up to a drunken indiscretion. He leaves the country in 7 months and in his words,"theoretically" will return with the girlfriend. He could just be freaking out about getting married. However, in the same conversation that he told me that he would ask her father for permission, he told me had it not been for his gf, he'd totally go home with me--which was VERY perplexing. Thoughts? Leave him alone, right?