surely this admission IS a form of harm isnt it??
Well, no. Thinking about sex with someone doesn't harm them.
If he's looking at explicit material featuring underage girls or if he's doing anything at all to approach, groom, spy on, touch or expose himself to them in any way whatsoever, then obviously he's harming them. But thinking about teenage girls and having a wank in private doesn't, on its own, harm them. It's absolutely foul, certainly, and he's a horrible creepy piece of shit. But what he has admitted to doesn't constitute harm.
However, obviously you need to dump him pronto because he's absolutely repulsive and gross. And for all we know he might actually be doing or have done all the things I listed above. I think I'd take steps to find out if he has any criminal convictions and I would also be inclined to have a look at his phone/computer/social media to see if there was anything on there that needed reporting to the police.
Does he have access to teenage girls through work, family, friends etc? The thought of, eg, someone like him working in a school or running a youth group or working in retail and managing teenage Saturday girls etc is making me feel a bit sick.
It's also incredibly weird that he actually admitted this to you. I mean, it's fortunate that he did, but it's bloody odd for a man to announce to his partner that he wanks off to thoughts of schoolgirls.