I agree with Debunkt also.

Instead of asking her "Do you want to have sex?" get her in the mood. Since she has been depressed, pull her away from her work for a night and do some nice things. Decorate a room with some dim candles, nice scent, and flowers. Give her a massage and/or bubble bath. If you get her in the mood, you won't have to ask.

I know that it is frustrating for you and I've actually been in a similar situation. I found that the worst thing to do is to give an ultimatum or threaten breakup if she does not give you sex. Being physically active is very important to some people in a relationship and I do agree that it is not fair for you to sacrifice not being pleased for what she wants.

Think of things you can do to get her in the mood. Ask her what she likes and dislikes about sex and maybe you can find out how to improve it.