When you are young, it's deeply annoying to be told that certain things are a condition of your youth. There's almost always some condescension in the proposition that your reality, your hopes, your frustrations, are just a condition of your age, that what feels unique to you is a very common thing after all.
Kids are the ultimate trump card: a way to get out of co-op board meetings or lunch with a friend you don't want to see or your brother-in-law's set at a comedy club. It's fair to use your kids as an excuse to sidestep what you don't want to do; it's less fair to blame them for not being able to achieve what you do want to do.
I think that in the cultural imagination, motherhood has a primacy that fatherhood just doesn't; and that's not to say that there aren't many fathers who are active and engaged and for whom that is their life's passion. But somehow, in the imagination, there's something different about maternity.