It's not NTR because the MC agrees with her seducing the girls, so it's not supposed to cause any jealousy. Other than that, if the scene is like you described it (I don't intend to play "Katrina's bitch" path, not even out of curiosity, so I'll just have to take your word for it) some people probably loved it, but for others (me included) it definitely sounds like cuckolding. It's not the first time I've seen the "I banged the girl you've been flirting with, but if I send you a video of it, you should find it hot" trope being used in a game and since it's girl on girl everyone has to like it or find it playful, but that's a different story for a different thread...
The thing is, if you choose to let her seduce the girls, you're basically asking for something like that. I don't think that path will get more love from the dev (Katrina has a scene with Gianna in that path, the MC has a scene with her in the other), though it's too early to tell for sure, but I've already said (right after the first release) that Katrina's offer sounds more like she wants to build a harem for herself and she's willing to give the MC some access to it (if not making him a part of it too...), if he's a good boy of course, rather than building a harem together. I know, she's more experienced and being a manipulative bitch is always a plus in these type of situations, so it makes sense for her to have some kind of an upper hand, but like you said she could have easily involved the MC too if she wanted to or at least give him a little something to seal their "partnership".
Anyway, at this point I'm more interested to see if the new girls are going to get more screen time in the next update and if there are more of them to be introduced in the near future.