To me gender isn’t isn’t relevant here, even if the kid is way older. The violation of privacy however is.
I don’t recall the age I had to teach my kid not to film me taking a shower or a dump. I believe by the age of 5 they had their own mind when they wanted to be filmed/have their picture taken.
Just to be clear by the age of 3-4 a child should be aware of the concept of not taking what belongs to others including their pictures.
If your SO was fine with their phone being used to take nudes of them it would not be an issue. However in your 1st comment you state they are not.
The kids is old enough to understand boundaries and the word “no”. If that behavior is limited to your own household then fine you do you. It never is though.