regardless of the moral issues, it happens all the time, so don't feel guilty about it. The other concern is your girl. Typically there are only a few reasons a girl gets like that.
1. She is depressed and miserable about some aspect of her life.
If that has to do with you or your relationship, fix that and sex returns. Sometimes it has nothing to do with you, but cheering her up can = booty time.
2. You guys aren't compatible and it took you some time to figure it out.
This happens all the time in younger couples because they don't realize that what they think they are looking for isn't what they want and now that they are stuck in something they hate it. If this is the case for either of you, save yourselves both the trouble and end it as fast as possible. The reason doesn't matter, but wasting time on something that isn't going to work is always better than the aftermath of forcing a square peg in a round hole.
3. She doesn't feel good enough.
This is a self esteem/confidence issue for her and you are a reminder of things that make her feel bad about herself. Could be through no fault of your own. Breaking up with her will be devastating, but putting up with doing everything right and still not being able to please her is even more agonizing (been there). Better to separate yourself until she grows up a little and gains some self esteem.
4. No longer interested in you but too chicken to break it off.
A LOT more girls do this than will admit. If you have even the slightest sense of it, she is just waiting for you do to end it, so do it and get it over with.
If you are pretty sure #1 is the situation, just talk to her. Often times expressing your frustrations can make the other person realize they don't have to do much to make things happy again. She may not even realize it. However, if she doesn't feel any sense of wrong doing for ignoring you sexually, she most likely falls into options 2-4.