Why girls sometimes don't wanna swallow - you know how some girls are at their early to late teens, they're very iffy on anything sexual. They have to lose their virginity to a very special person, you need to wait six months in the relationship, their self conscious, they also can think that there is some power drawn when you go down on a guy, the physical level you're on when you're kneeling down, or basically below him. Not that all of that is a bad thing, I admit to some of that, and other girls are like that too, and they can be iffy on the concept about swallowing. Or that it tastes bad.
Why guys find it a turn on? - I really don't know. I guess the alternative - spitting it out in comparison to swallowing it. I'm only guessing here, but maybe it's that acceptance feeling - like what if your partner decided to pull away while you're kissing and spit because he didn't want to touch your saliva or whatever. I dunno. I really have no idea.