(and I also agree with the pp that mom should get an apology for the 'confrontation' since, IMO, she did nothing wrong..)
My older dd was 4 when I became pregnant with her sister. And she was one who wasn't satisfied with the simple "sperm meets egg" story -- she wanted to know specifically HOW it happened. To be exact, she asked if she could watch us, and I explained that sex is a very private thing.
But even though I wasn't willing to have sex in front of her, or show her videos of other people having sex, I of course wanted to do my best to satisfy her curiosity by giving her accurate information. I also did say that not all of her friends knew this stuff yet, and that their parents wanted to be the ones to tell them when they had the questions.
And it never really came up with friends or friends' parents, or at least it was never brought to my attention. But as far as withholding the information until they ask the questions -- my current 4yo isn't as verbally-advanced as her sister was at this age (plus I'm not pregnant), so she hasn't directly asked these same questions yet.
However, she HAS seen our female cat going into heat and mating, and then going through pregnancy and giving birth. She's also seen bugs mating. I'm not sure if she exactly connects the sex with the babies at this point -- but she enjoys watching her own birth video, and periodically asks to see this or other "babies coming out of mommies' butts" which we can easily find with a quick google.
She loves reenacting her own birth, and also having her little toy animals and her dolls give birth.
It seems more surprising to me when I hear children have reached puberty without ever having expressed any curiosity about sex or reproduction -- but I guess it can happen. Still, when parents say they don't want their children to discover anything about sex until they think to ask the questions themselves, I can't help wondering if they plan to keep them indoors 24/7 and keep them away from all kinds of nature study.
Because sex is all around us. It's the stuff of life, you know. The other day my oldest saw flies flying and mating at the same time. Cool stuff.