As a man, I find the idea of saying it's natural to find kids attractive highly offensive, so I'm going to have my say.
Firstly, this is not a porn argument. If it were, it would be down to OP to decide what she will and will not accept in a relationship, with regards to legal porn. But I don't think that's relevant. As has been said, "teen" porn is 18-25 pretending to be younger. Which is still quite disturbing when you think about it, but it's not really relevant.
OP's DHAKA sounds like he couldn't get off on that fantasy and has instead turned to real teenagers instead. Which is far more disturbing than if he'd been just jacking off to porn.
Also, on the subject of it being normal for men to find girls attentive. I can't on any level agree. Yes, when I was 15, a 15 year old was attractive. But as I grew up, so too did my taste in women. As a 29 year old, I don't think I could see an 18 year old as being much more than a child. I couldn't possibly look at 15 year old now and see anything more than a little girl.
As for the reference to history used to justify pedophilia? Yes, in the middle ages they married girls off at 13. For one thing, they also burned people at the stake and held public beheadings. We don't do that any more, either. For another, thre reason they did that was because of the high infant mortality rate. The thinking was to start having children as soon as possible in order to ensure survival. We now know it is damaging to their reproductive system and probably contributed to the mortality. There's nothing to say the men actually found a l young girl more attractive other than the erroneous belief that she could give healthy children. Or that any attractiveness wasn't society telling them to. For example, in some countries an older women is more attractive. In others, a larger women is a sign of wealth and therefore attractive.
There's no excuse for jacking off to teens. OP is not over reacting and this guy is clearly not normal and may be of serious harm to children, IMO.
The law is place to protect children from harm and because until they are older, children don't have the emotional maturity to enter into sexual relationships or to raise children of their own. I see absolutely no reason why any normal man of middle age should be finding young teenagers arousing.