You are not sinning by not having kids now. But if you attempted a marriage with this “feeling” and a permanent intention against children, then yes you would be sinning.
As dating leads to marriage, and marriage leads to children, it is better to not do any of those things unless you are open to the final end of the process.
I can say that I had no desire for children when I was a teen and young adult. But I started dating my husband, and feel in love. He wanted the whole domestic thing. I had to discern whether marriage (as the church defines it, which is a whole lot different than love that person and that makes me happy) was my vocation, and had to accept the fact that if marriage was my vocation then children more than likely was too. How unfair to my significant other if I continued to date him and not accept the ends of Marriage (that the Church defines and he wanted). I am now a mother and take great joy in my vocation.