No it’s not. “Hey, I really like you, but you’ve had too much to drink and this wouldn’t be right. If you feel this way tomorrow, let me know,” is a perfectly acceptable answer. Hell, I’ve given that answer before. It’s certainly not less tactful than, “You have a boyfriend and what you’re doing is wrong.” And yeah, cheating on your boyfriend is bad, but not as bad as rape, which this would have been.
Keep in mind, you’re telling this to a blackout horny drunk person. In honesty, it doesn’t matter what you actually say.
OK, so if it doesn’t matter what he said, why lie? You’re kinda invalidating your initial point here.
Well, okay, the drunk person will still react to what you say; they just won’t remember. Having a gracious exit to the situation is preferable to, say, “OH MY GOD THERE’S AN AXE MURDERER BEHIND YOU” and running away when they turn around.
I began with ‘have some tact’ after all.