Of course I don't think he should be seeing random naked adult women either. The swimming pool we go to has individual cubicles for everyone so this never arises. Not sure where you're going.
The issue isn't with seeing it in a safe context like a book or lesson about the human body, it's with him on his own in a situation where adult men are allowed to get their penises out and therefore an opportunity for any creeps. When his dad's with him obviously they go in the Gents as he's there to protect him if needed.
Yes I know that the statistics say children are more likely to be abused at home but I am not primarily worried about rape or sexual abuse in these cases but about "lesser" but upsetting situations like flashers or inappropriate comments or brief touching. I am willing to bet there are not many statistics about the prevalence of this, but every woman I know experienced this in childhood, I think it's likely quite a few men did too.
I also - to respond to someone else - never said anything about the age of 8, I said that eventually he'll refuse to go in the Ladies and I hope that this is at a time when he is able to handle upsetting/difficult situations if necessary, not as a tiny wee 7 year old.
And speaking of that, the idea that my lad is making any grown women uncomfortable in the Ladies is just silly. Or even small girls, since we all have cubicles there and I'm always with him.
I would like to genuinely thank those who have posted sympathetic suggestions. But given the length of this thread, it has mostly been debate and that's not what I asked, I'm happy with my choice, just wanted ideas for how to explain it so he understands my decision, without frightening him. I suppose I should have known that on Mumsnet mentioning public toilets would be an invitation for the people obsessed with debating trans issues to chime in, way to live up to this site's reputation. It's really boring though that everything always comes back to this here.