The girl(s) are just another symptom of the problem. It's not that she is cuter, nicer, more interesting, blah blah blah than you. He probably likes the fact that he can drink all he wants to and have the company of a woman who (in his mind) is finding him witty, charming, attractive. He knows that if he sat next to you drinking like that you would just be disappointed and disgusted. You know him too well. I've learned that they already think of themselves in a very, very negative light and need to seek out people who make them feel special. That's my guess as to why substance abuse often is accompanied by cheating or flirting.
Just know that this is about him and his self esteem, or lack thereof, and not one ounce about you. The affections of others are just like a drug-a temporary escape from reality.
I'll grit my teeth for both of us tonight! Hang in there.