Wollstonecraft thinks that a parent's affection for their children is actually just a blind form of self-obsession. In other words, parents who think their kids are perfect are thinking, deep down, that they are perfect. Mary also refers back to her earlier argument about parents being like tyrants, demanding blind obedience without ever wanting to justify themselves.
Wollstonecraft also thinks that parents are extremely selfish when it comes to their kids, because once they have kids, they don't care about anyone else in the world. Family actually breaks down community more than it supports it.
The fact is that a good mother has to be able to think for herself and be somewhat self-aware of how she raises her kids. Otherwise she'll just become another overindulgent mother who raises her kids to think that they're the center of the universe.
At the end of the day, there has to be a good relationship between parents if the children are going to learn how to properly treat others. And the bond between parents must be based on friendship and not just sexual attraction, because the second one will wither over time.