It doesn't matter whether other people are ok with this, it's what you think that matters, everyone's boundaries are different and a good partner respects those boundaries.
From your posts a number of things are clear. Firstly, the positive, he's been open about it with you, so isn't actively trying to deceive you. That's important.
Now onto the bad. He's letting you do all the work with your baby at night whilst he has time to wank to strangers having sex. That's so unfair and he needs to do more. Also, I wouldn't imagine this is one of those things that men justify doing because "all men do it", this is a few steps beyond that and he should have had the decency to check whether this is something you're ok with first (he must have known there was a distinct possibility you would not be ok with it, seeing as he has hidden similar stuff from you in the past).
Lastly, if he's spending family money on this, he certainly should be checking with you first.
Even if I were comfortable with what he's doing, I think I'd personally be worried about what else he's given himself permission to do, as he seems to be struggling to understand what you are and aren't comfortable with in terms of his internet use or he fully understands and doesn't care