The poor OP's situation just shows how we end up splitting hairs if we try to accept "regular porn" because then men will just say " whats the difference between that and a cam girl?" And then from cam girl to strip club to lap dance.
I agree in one way, but in another I think many people have a line. They appreciate the difference between pre recorded, professional porn ..& interactive porn, purchases, seeking local sex workers etc and don't cross it. They don't need told that or tiaught that, it's obvious to them.
If they don't have that line, the tempting thing to argue (and that they will usually argue) is that they were somehow clueless that the cams/purchases etc are different. I think in most cases that's a bit of a lie. In most cases they are the type to take the piss, they are dishonest, they are the type to push things.
(We already know op's p is that type from his previous behaviour that op has now described.
He has a history of serial cheating in his previous significant relationship (he wasn't happy, it was shit ...so, just finish it!), He has repeated incidents of going AWOL drinking with mates and turning his phone off and staying out all night - op relates that she doesn't believe his story about one overnight when he apparently slept on a bench, he's tried to gas light op that her reaction to this behaviour was abnormal and controlling, he's told her he doesn't want her checking his spending on those occasions. In a ft relationship with a baby on the way she's found him purchasing only fans videos & images and searching only fans workers in their area, she's now found out about the cam sex . She's crazy and ott in her reactions to all these things apparently. There's a bit of a pattern here (!))
Back on topic, there is a tiny chance I suppose (??) that in some cases it is due to real cluelessness, lack of empathy, lack of emotional intelligence, dumb-ness ...but even if that's true; who wants to be in a relationship with someone so lacking in empathy and emotional intelligence that they act inappropriately and hurt their partner.
I don't actually believe they can be that clueless though, I think they know what they're doing is not kosher, but they think they'll not get caught and if they do get caught they'll just roll out all the minimisation and blame and manipulation tactics that we've seen op's p so (and every partner on here).
Usually, if pushed they'll go for MH problems, sex addiction, depression, not getting enough "attention" etc.
They know they're doing wrong, they know it's not really right ... They don't clarify if it's ok with their partner for a reason; because they want to do it and feel entitled to do it.