For me (I am a girl), I think it's all about the extension of the male orgasm. Also I always liked watch guys masturbate, and what is more hot is to see a guy (or guys) masturbate to the sight of me. That load of cum and how much of it that shoots out are more or less because of "me"...and how could you not like that?
As for watching bukkake, I love to be able to picture myself in the position of the girl. Also for me as girl there's alot of jerking and a lot of cocks to watch. Me like!!

Pretty much agree.