"Because when we marry we exchange the right to marital intercourse, in an absolute way. We are not to withhold it from one another. St. Paul even discusses this in 1 Corinthians 7:
The husband should fulfill his duty toward his wife, and likewise the wife toward her husband
A wife does not have authority over her own body, but rather her husband, and similarly a husband does not have authority over his own body, but rather his wife. Do not deprive each other, except perhaps by mutual consent for a time, to be free for prayer, but then return to one another, so that Satan may not tempt you through your lack of self-control."
I think the issue of whether her "withholding" is a sin or not stems from the reason for her refusal. She may be angry (doesn't sound like it, from your post) or she may have physical issues that make intimacy uncomfortable or painful.
Can't help thinking of that old Woody Allen movie, where the couple separately discusses a question from the therapist as to the frequency of their sex life;
He says - we hardly ever have sex, maybe three times a week?
She says - we have sex ALL the time, like three times a week!!
I'm not making light of your issue, I'm just saying that with increased communication, love, understanding and prayer you should be able to get to a place that is good for both of you.