I was wondering if it's sinful for a woman not to ever want to have children?
A person has the right to choose to get married or not. That choice comes with consequences since sex is only allowed within a marriage. "But I say to the unmarried and to the widows: It is good for them if they remain even as I am; but if they cannot exercise self-control, let them marry. For it is better to marry than to burn with passion" (I Corinthians 7:8-9).
Decisions about whether to have children, how many, and how soon, are decisions that should be made jointly by the couple. Marriage is a partnership where the two are one. "And He answered and said to them, "Have you not read that He who made them at the beginning 'made them male and female,' and said, 'For this reason a man shall leave his father and mother and be joined to his wife, and the two shall become one flesh'? So then, they are no longer two but one flesh. Therefore what God has joined together, let not man separate" (Matthew 19:4-6). There is a severe problem when the wife or husband alone decides family matters.
Mankind was given dominion over the world. We regulate and alter the natural course of the world around us. We plant fields and not just gather wilds produce. We build homes, make clothing, and in many ways control the world we live in. Decisions about when to have children is a part of that process. Acts of sex do produce children, but not always. Sex isn't exclusively just for children, it is also mean to be for the enjoyment and needs of the husband and wife (Proverbs 5:18-19; I Corinthians 7:2-9).
There are times when having children would not be wise. For example, when a wife is having health problems, when a couple is older and a pregnancy might risk the woman’s health or carry a high risk of producing a defective child, or when a couple is not financially able to support another child. Sexual intercourse is required in a marriage and avoiding sex simply to avoid having a child is not right. Like all authorized activities the control of the production of children can be misused. However, because some misuse a right does not imply that a practice is inherently wrong.
So it really comes down to your motivation as to why you don't want to have children.