hey thanks bro! 

correction: they were not my friends. after we had sex then we became friends. and I don't have sex with them regularly.

regarding masturbation, girls also do that. at least some of the girls who are close with me do that.
i have a friend in another state who called me while she masturbated (which was annoying, since she did that when I was *at work*)

my advise is simple: ASK HER.

the idea of having sex is not to satisfy yourself, but most importantly, to satisfy your partner. sometimes you and your partner don't know that.
you can talk about this with her, heart-to-heart. be honest about what you want, and be willing to help her to satisfy you.
of course, be fair, you must be able to satisfy her too

let me give you example.

when i asked my girl to do anal, she was upset. she thought, "you think my V not enough to satisfy you?". actually i just wanted some variation! but i asked it wrongly. later on, after we're more open to each other, she voluntarily said, "hey, you want to try anal?" she tried everything to satisfy me. in return, i tried everything to satisfy her.

Well, it does sound like she's a selfish lover...

But first, consider improving your own skills first. If you want her to be more proactive, you need to really get her aroused. Then only she'll be semangat.    It could be that she's never really known how good sex can be - a lot of Asian/traditionally-minded women have that problem.

Learn how to give her an orgasm. And learn how to tell if her orgasm is real. 

You're doin it wrong maybe, or perhaps she is not ready for it yet? You have to understand.

My ex was also "lazy", but she might be uneducated (about sex). I tried to educate her, but she's very close-minded and conservative (and you're wondering why she had sex with me at all). dexterhau, I have to say, based on my experience, it might be not your fault. If you have tried hard to educate/inform/teach her and she persists, either you have to accept her the way she is, or leave her (so both of you can find suitable partner).

Oh, one more thing. You girlfriend behaves like that can also be caused by her feeling. She might feel guilty (explain to her!), she might found you not attractive enough (shape up your body!), or she might do that just for the sake of "because I have to do it" (tell her!).