I have two angles to approach this problem. The first one is the most obvious: it is wrong to hold out on your spouse. It is a mortal sin. If you want to end you rmarriage or cause a separation, there is no faster way than to do this to them. Sometimes it's because of infidelity. Other times, they just don't want to do it, which is amazing, considering our sexed up culture. For Catholics, these days, it seems that NFP can be abused, as there is a fear of getting pregnant. But Church teaching is incontrovertible: even NFP can be abused, and a spouse should never refuse to do this. Listen to the following:
The Sanctity of Marriage: The Duty of Motherhood vs the Abuses of NFP
[youtube]https://www.youtube.com/embed/mSY7FqmJ7js[/youtube]
On the other hand, perhaps things are not going smoothly on the homefront. Sometimes she does this because she feels insecure. Perhaps there's not a steady paycheck. Perhaps a lot of the burden for keeping the house in order rests on her shoulders, and she's terrified of added responsibility of possibly getting pregnant. In such a case, that is no excuse whatsoever, but helping more around the house or doing something to ensure your spouse feels secure about your family's future may be a possible avenue for you to instill confidence in the household.
This is not a nice situation to be in, and a lot of people are embarassed to have this issue. It is more widespread than you think, and it's a shame that people have to put each other through this kind of an ordeal in a time when stable marriages are so rare. All the more reason to do your best to try to crack the code and make it work.