IDK man, if I had a time-traveling, giga powerful cyborg friend I wouldn't sit and watch.
Ah, but the MC doesn't know he has a time-traveling, giga powerful robot. That's what's cool about it. He thinks he's got a cool sexbot and he's finding out about her, but the player knows she's much more. So the player knows the decisions being made are much more than what the character sees. And the rest of the sex isn't just about the MC getting laid, it's about developing the characters so that you don't end up sitting and watching (unless you're into that).
I don't want to get too deep about it, it's just much more interesting than "Here's the scene where you choose an optional fetish". Whether you want to see the NTR or not, it's actually relevant to the player's choices as far as the story goes. If you don't like NTR then you don't have to see it, but the "threat" of it is what makes Melvin a bad guy and makes your choices relevant to the story.