Children love asking questions. They do it out of curiosity and out of innocence. They genuinely want to know; there's no judgment behind it, and nobody gives a lecture here.

The child asks a woman why she has hair on her legs. She says, "I'm a mammal," in a rather playful tone. She posted this story on Reddit under the username goldencain1410, explaining she wasn't prepared for questions, so she answered whatever came to her mind. The child just went away a little confused. We could just laugh at this conversation, but the kid's mother had a different approach. She wanted to know what this woman was "teaching her son" and accused her of trying to parent him. The woman never even attempted to do that. The woman's response didn't tell the boy what he should believe about shaving, what his mother should do, or how anyone should dress. In fact, if the woman hadn't answered anything at all, the mother would have gotten upset all the same. 

But his mom followed the accusation with her shaving schedule and asked the woman what her excuse for not shaving was. Why would she be forced to explain her bathroom routine to a stranger? In what way is not shaving something she had to excuse herself for? She wasn't asking anyone to approve or disapprove of that decision. She was there to celebrate her best friend's daughter's birthday. Answering a random child's harmless question didn't make her his parent, and having children doesn't make somebody else's leg hair your business. 

Anyways, the guest kept her distance afterward for the rest of the party, which seems a reasonable decision to stop a conversation that never needed to become an argument in the first place.