If any guy is "repulsed" by an "ugly" girl asking him out, He doesn't deserve any affection. Yet, beauty is in the eye of the beholder. or One man's junk is another man's treasure. So an appropriate question would be. "What do you think of an unattractive girl who asks you out?", In which case, I would state the simple "sorry, I'm not much of a relationship person these days"
i would be polite and depends on how ugly she is. if she's super ugly I'd probably make up a nice story to get out of it.
I wouldn't be repulsed. If someone is attracted to me, I see it as flattering whether she is beautiful or ugly or if it's a gay guy. lol I'm not picky about who finds me attractive. That would be pretty douchey.
If I wasn't attracted to her and she asked me out one on one, I'd just respectfully decline her offer.