Because he was very drunk. People who are very drunk do very dumb things. His foggy, alcohol laden brain only could think of how much you didn't like alcoholics and reacted on first impulse to hide it.
What makes him a man worthy of respect and adoration is that he remembered it and was honest with you when not smashed in the morning. This is a sign that he is an emotionally mature adult who is accepting responsibility for his mistakes.
Be ecstatic that he is capable of this and proud of him for fessing up. Keep this one. Unlike many men, he cares about the truth and knows what self respect is and when he has violated it.
Oh, and REWARD this behavior!!!! You want to encourage him to keep being honest with you when he realizes he's screwed up. Seriously, thank him and treat him to something. Set your personal fears and baggage aside (or get counseling for them) and reward this man for doing exactly what he should have done when he realized he blew it about something.