Why do boys get off with ugly girls?

Do guys go for 'easy' girls because I cant understand why! but theres a group of sluts that arent even that pretty at my school but they manage to get off with everyone! So do guys ever really properly like that kind of girl? Or do they like a...

challenge (ie - the shy girl or more 'hard to get' girl AKA me!?)

"Easy" girls get used. They dont actually get the guys, they just sleep with them. That's why most guys pay attention to them, because they sleep around. They do not actually think about dating these girls, guys date more real girls, girls that dont sleep with everything that has 2 legs, girls that put up a challenge, girls that excite them in other ways.
The "easy" girls are dumb, they are all being used and they dont even know it. They think they have all the guys, but in reality, all they have is a bunch of one night stands.

I can tell you from experience that easy girls, ugly or smoking hot are a big turn off. I have had too many encounters with easy girls, it is never anything I wish for. I recently had some very strong feelings for a girl in one of my classes, this was up until we met up outside of class for the first time, she and I hooked up. It was a big turn off for me and I decided that it was enough to change my mind about her. Easy girls, or sluts are never what any guy wants in a girl, unless he just wants sex. A girl who does wait to get more physical, a challenge can be a real turn on. Also it makes it so that when something does happen it means a lot more and it feels a lot better.