When she told her son she had sex once or twice a week with his dad, he immediately told her to "never say that again".
"Am I being unreasonable to be mortified at this question from my son?" she wrote on Mumsnet on Sunday 17 September.
"I am a very modest person and never talk about sex with friends at all, but my son has far fewer inhibitions and often mentions things I find embarrassing. I would never in a million years have asked that question of my mother.
"Even though I tried to seem calm and cool, it felt just really odd. Maybe because I was taken by surprise."
The mum asked whether any other parents had conversations like this with their children, admitting she never expected such personal questions from her son.
Mumsnet users were in two minds it: some felt it was great the son felt comfortable asking his mum questions about sex, but others believed it was too personal a question to answer.
"To be honest, he asked and you felt ok in answering," one person wrote. "It's not as if you casually mentioned it to the cashier at Tesco or something. It's typical that he'll feel grossed out by it. But there we go. He asked a question and got an answer."
Another wrote: "I can't say I've ever asked my mum this, but I know when I was learning about sex I wondered a lot about things like this and how much people were having sex.
"I'd say it's a credit to you that he feels comfortable to ask you the question, even if he didn't like the answer."
Someone else felt that the 12-year-old "crossed the boundaries" in the mother-son relationship, writing: "It is inappropriate for anyone to enquire about your sex life. Be that your parents, children or friends. Some things just are private."
One person also wrote: "Did you ask him why he wanted to know? That is a very personal question and to be honest I would've probably told him it was none of his business."
What do you think? Would you have answered your child's question about your sex life? Let us know in the comments below.