Why do women assume that a 9 years old boy doesn't know anything about sex?

I was visiting my relatives at that age and one day my aunt took me with her as an excuse for my uncle in order to visit her friend without my uncle.

Then, the other woman after which could have been a couple of minutes of attending us with drinks and snacks while asking about the family and such and she started with her conversation.

She was describing how one of their friends in common caught her husband sexual act with another woman. Then, after some seconds that my aunt wanted to know more, her friend asked my aunt if it was fine to talk about sex in front of me.

She said: "Can we keep talking in front of him?" "is there no prob...?" So my aunt said that: "it's all right" that I didn't understand anything, "he is only 9 years old"

Her friend described the man's member because my aunt wanted to know exactly if he committed adultery completely. But after saying about the fight she worried about me as a prudish person that she was and didn't mention more of the sexual part.

I realized that day how many women who wear a conservative mask like to talk about sex when their husbands are not near.

At school we had a very basic info on human anatomy plus what one watch on TV or reads is more than enough what a sex act is or what means infidelity.