I believe that alcohol is a poor excuse for bad behavior.
A person who is capable of cheating on you drunk is capable of cheating on you sober. That's not a line you just happen to accidentally cross because you drink too much one night.
I'm not saying cheating has to be an absolute deal breaker for everyone. But I wouldn't tolerate "I got so drunk I don't know what happened" as an excuse. That would just mean we would have TWO problems -- drinking AND cheating -- instead of one.