My ex did this to me near the end of our relationship. I went along with it under the misguided impression that it would actually mend whatever issues were between us. How wrong I was.
But in her defense (she'd double-over with laughter if she ever saw this), I wanted more sex than she was really comfortable giving. It was a bad scene and there are days that I'm glad it's over. Now I just need to find someone who is comfortable with the level of sex I'm interested in.
I agree with some of the others here that withholding sex is not a good thing, unless there is enough communication and understanding to justify it.