If you've been childless for any length of time for any reason, you've probably encountered this, and I've noticed that it increases in frequency as I get older and more of my peers have children.
For whatever reason, there is the misconception that because a woman or couple does not have children, they automatically have more time, availability, resources and/or flexibility than mothers do.
While I won't deny that a mother's job is hard, time consuming and sometimes overwhelming, all in ways that I can't understand, her time and her efforts are not more valuable than those of a childless woman.
If a woman or couple does not have children, it is likely that they have filled their time and financial budgets with other activities and priorities and her obligations are as important to her as the mother's are. Those obligations could be any number of things and they may or may not be flexible or optional.
I won't argue that some childless people are footloose and fancy free and have all the disposable time and money they could want. But I'd also argue that there are parents like this too. A person's work ethic in whatever task they are called to is not determined by her biology.
Such a mentality indicates a frighteningly low value of woman. It purports that a woman does not have value (or at least not enough to be given any consideration by her peers) until she has children. May it never be that we deny the power of the completed work of God in a woman's life before she conceives!
Besides, when someone makes a comment or assumption like that, it often just makes the infertile person want to scream, "but don't you understand? I would give anything to be overwhelmed with soccer practice, laundry, and homework!"