It can be, if that constitutes defamation or libel. A passing statement wouldn't, but a post on a popular website absolutely could. It all comes down to the damages that (false) statement caused.
If the person is a slut it wouldn't be libel but it would still damage reputation. The person being a slut is true but calling them one still damages their reputation. If you release a home made video of a pornstar it would still be illegal even though it's not something that would damage their reputation.
The reason for the illegality is the lack of consent not the reputation damage.
That depends on whether there was a reasonable expectation of privacy. If it's in public, there's no reasonable expectation of privacy.
Even in a 1 party consent state recording someone while you are having intercourse with them is illegal without their consent, because we make exceptions for especially sensitive subjects such as sex.
To go along with that I also believe that people who uploaded photos of themselves/their children did not consent to having their photos used to make sexual content. If they did it would be another matter to me entirely.
Edit: I also would like to say (and I really am sorry for bringing them into this) but from what you said you think it would be okay (not socially acceptable but okay/fine) for someone to take pictures of your kids while they're at the park and use that to make porn. Really think about that. Is that something you think should be allowed? Imagine someone taking pictures of them at walmart and you ask what they're doing and they straight up tell you "I like how they look I'm going to add them to my training data to make porn, don't worry though I'm not sharing it with anyone" and you could do jack shit about it without facing legal consequences yourself. You think that is okay?