Is being drunk a valid excuse for cheating in a relationship?

You could have put down the bottle and you didn't. You could have made sure you were not in that situation and you did anyways. Also not everyone cheats when they are drunk. Some people behave them selves no matter how drunk they are. While yes it would happen if you got drunk it is not an excuse. However if someone gets you drunk to take advantage of you knowing you wont be able to keep them off that is their bad not yours.