This game's MC is what is called an Alpha Cuck, basically unlike the typical NTR story with a Beta Cuck MC, the Alpha Cuck get to fuck around just as much as their partner, if not more, so basically more along the lines of a typical NTS story MC. In this case its clear Erik is the big dicked Chad that all the girls lust after but he isn't too possessive so he let them get fuck around. At the end however, he clearly is going to be the one reaping the girls, I mean Ari has effectively already jumped ship to him being her main dick, Anya has a big thing for him, Maki hasn't quite made the jump yet, but its clear she is very interested in playing with him.
I think you are projecting a bit, yea sure Erik says 'he knows what to expect', however I feel you confused that with what you expected, but have you watched what he thinks while fucking Ari and seeing the pictures? He is clearly shocked at it, its clear he expected her to do less than she did, he clearly think she went too far. In this you confound pushing boundaries with jumping over them, so it make sense for Violet to have more restrain than she showed originally, like I said it felt like she jumped over some steps of 'boundary pushing' in the process in the previous version, and if she doesn't want to mess up her relationship with Erik going slower is sensible, she shouldn't be jumping from being exclusive to a slut like Ari in a snap of fingers, she is supposed to develop into over several events.
Hell the only reason he was likely good with it at the end was because he got emotionally confused by being ball deep in another girl, if he had been shown those pictures without that, he might have been furious. Your assumption here is that because he said he expect her to be naughty that means he is good for everything and that Violet would have taken it as him being good for everything, that isn't necessarily the truth, like I said not everyone is on the same level in such relationship, for Violet to risk like I said jumping a few steps was to take a risk Erik would be in a foul mood because of it, which since she loves him she wouldn't want that for him, she wants him to be good with what she does, so more restraint make sense.