Maybe they're dating those "butt ugly " guys because they treat them extremely well. There are guys ( not all , but some) that can have any girl they want and their not oblivious to this fact so they collect girls . Sometimes it's nice to be with someone that 's not only interested in dating you and then putting you on the shelf . These guys treat these girls well and for that ( some of them ) are rewarded in the best way with LOYALTY and ADMIRATION.
Besides, beauty is relative and we all look for different things. Then again, some girls do date guys who are less attractive to boost their own self esteem... but looks aren't everything and it could be because they actually like the guy besides his appearance.
LOL! I ask the same question when I see unattractive girls with super fine guys. I have just come to the conclusion that beaut is in the eye of the beholder and that guy may very well be attractive inside and out to her. For the others (not me), they're probably just settling because it's hard out here for us girls to find someone we are attracted to inside and out. I refuse to settle because I don't want to end up unhappy like some friends of mine that have settled.
I would say its because their personality. They most likely are looking beyond their outer appearance and their personality is what makes them look good. Some pretty girls go for guys looks first and some of them have some jerky personalities. Maybe they get tired of it and just want to start being treated right and start dating guys that have great personalities and don't care about their looks. And its the same for cute guys to. I see cute guys with not so pretty girls but hey if their happy that's all that matters :)