The reason she'd regret the latter is because society would shame and bully her for it.
Sometimes morals are the devil, and it is so in this case. Logically, there is no physical or mental harm, unless others choose to inflict it as a result of doing something they consider immoral and want punished.
Even in the case of CSAM I read this is often used to blackmail the child into continued exploitation. They'll get one photo out of pretending to be someone of the same age and all that, and then they'll get more by blackmailing the child that if they don't they'll send the photo to their parents, friends, and all that. The child has already learned that what they did was wrong and that people will make a big deal out of it. They fear the moral repercussions more than to continue along with the exploiter. That's messed up.
In my opinion, this same morality is also responsible for the illegal market around sex work and its bad conditions. Because it puts the workers and/or the exploited into a corner, with everyone against them, on one side there's the exploiter, and on the other side the moral police. That makes it that sex workers have no way out.
If sex work wasn't moralized, and was just work like any other, you'd be able to see the clientele improve, since more normal people would pay for the service, where as now it's only people willing to break through the morals that do so which tend to be sleezier. And you'd see the work conditions improve, as workers could mobilize, freely hop from one employer to another, go to the police when they need to report bad conduct or abuse, file a lawsuit against their employer, etc.
But it would take more than legalization, it would need to become morally inconspicuous and accepted as just another normal thing. Until that happens, well, yes, you're totally right, it would be regrettable to post a photo of your butthole online, as society will shame you for it big time. In fact, it doesn't care if you post it yourself voluntarily, or if you are exploited into posting it, or if your ex-boyfriend posts it out of revenge, you'll get shamed and punished by society equally on all cases for it. That's where I feel society is responsible, we create the opportunity for harm by making it shameful.