People are different. As for me personally I wouldn't have sex with my boyfriend after he was being a dick either. And begging and arguing about sex isn't exactly a turn on.
Yes, it's called being a prostitute. I know people, women in particular will be rather defensive about this and say that a prostitute is only a woman that openly offers sex for money, but don't kid yourself. I guess they're more commonly called gold-diggers but no woman NEEDS a man to buy her anything to want to sleep with him, sexual attraction is just that, sexual attraction, brought on my, well how do I make this clear, attraction. Attraction to an individuals smells or looks or intelligence. To me your girlfriend seems like a classic case of she's cheating on you.
^LMFAO But anyway, this sounds really immature... you seem like a jerk :\ I mean, i can understand that you are frustrated that she wont have sex with you so your frustrations are making you sound like one, but it's hard to tell since we don't know you ._. Maybe try treating her nicer. I wouldn't want to have sex with a guy if he was being kinda selfish like that, thinking about sex and not my feelings. I could be wrong and you could be doing things for her and being nice, and she is maybe using you. But it's, again, hard to tell because this is just a toast. ---edit LMFAO. I meant to write "This is just a post" XDD!!! I like toast too much ;-;
I say ditch her if she only wants to have sex with you when you do nice things for her. Or maybe she has some other shit going on she is trying to deal with and you should talk to her. Or maybe she thinks the only reason you are seeing her is because you want to fuck (kinda comes of like that in your post). Or maybe she has caught something horrible, it can happen. The whole "nice guy" thing : (I used to be a slight "nice guy" tbh) . Woman dont want to be treated like shit (in my experience), they do however want someone who is emotionally honest and somewhat confident or centred. Trying to weasel your way into their life as a friend or confidante, talking down their boyfriend or complimenting them on everything they do is the exact opposite of honest. Not expressing your feelings or opinions at any point is not honest. You are not being "nice", you are trying to manipulate them. I doubt you are even aware of it. People (in my experience) want a partner to share their life with, an equal, not a reflection. Also, in my opinion if you dont have enough happiness to share that happiness with someone else, then you should really be concentrating on fixing your own life.