I can certainly understand the desire to have sex but not have children. It may not even be that the woman doesn't want children, but rather that, for various reasons, she doesn't believe she is currently ready to have one, or more than one if she already has one or more. I think there's nothing wrong with that desire. This desire can happen in marriage and outside of it.
In and of itself, there's nothing wrong with that desire. This, in Christian theology, gets into the distinction between temptation and sin.
However, one must be VERY careful with such desires, as they easily lead to sin. The "Christian course of action" to take when facing such a desire is to immediately RUN from it (don't even entertain the thought). Don't even entertain the thought of sex outside of marriage. Don't even entertain the thought of sex when there's no intention of procreating. Don't even entertain the thought of watching porn and/or masturbating. All of those thoughts can easily lead to actions that will definitely lead to destruction/unfruitfulness rather than construction/fruitfulness.
While I'm a Christian and I like to promote my faith to others, this really doesn't have to be about Christianity at all. There's still a number of very practical and healthy reasons to view this subject matter in a similar way as I do.
I also don't mean to put myself on any sort of a pedestal either; If I deserve to be anywhere, it's towards the bottom of the barrel. I'm guilty of doing some of the very same things that I speak out against on here. I'm guilty of succumbing to various temptations. I'm even guilty of carelessly allowing certain temptations to become habitual sins (and experiencing firsthand how extremely difficult it is to loose oneself from such sins when it gets to that point).