Honestly, I cannot decide if it should be treated as a sex crime.
If it was child pornography, with no actual sex, most would agree it was a sex crime.
If someone else took the photos without her knowledge, like a hidden dressing room camera, some would consider it a "sex" crime.
Most would agree, if the photos were not about forbidden nudes, they would not have been taken and sold to the tabloids.
You bring up a good point that I had not accounted for. If the photos were taken without her permission, then the original photography may be considered sex related, to the same extent that peeping-tom offenses are.
But if the pics were consensual, the fact that an adult woman’s nude photos were stolen is about theft, not sex.