The first thing you know about Lucy is that she likes sex a lot, and in fact the MC knows it. And it's something they talk about and it doesn't seem to matter too much to the MC. That she doesn't tell him about the sex trip is silly when he knows that she's sleeping with guys just to hang out. I think it has more to do with the "curse", "spell" or whatever it's that affects her to feel at the same time jealous of the MC's possible relationships and reluctant to talk about sex with him, even though the MC knows about her sex life. Whatever it's, it makes her "ashamed" to talk about sex with him, even though she is incredibly attracted to the MC, while also generating an incredible amount of jealousy toward other potential "rivals." Which doesn't fit her uninhibited personality (which makes magic the most plausible explanation).
Personally I don't like NTR and I like the game, I can't really find it anywhere (Rose and Ellie's relationship is casual and temporary, in the words of the developer, in fact I suspect that it ends in the next update, but I'm not sure, I guess it will continue until Rose finds out that the MC has a penis) As far as I know, I think Ellie will only have casual sex with women (no serious relationship except with the MC). I don't think anyone tries to steal the LIs from the MC (even if they have casual sex with other characters, with none of them it will be nothing serious). For example, Lucy has sex with various characters (male, female, futa, and fairy), but she doesn't seem to care romantically about any of them (in fact if you read her thoughts she's clearly obsessed with the MC), because of the MC's personality, I don't think he's jealous that Lucy had casual sex with those characters (and what I don't like about the NTR is the MC's frustration at seeing how the LIs leave him, and that doesn't seem to be happening here at all: casual sex without feelings I don't think can be considered such a thing).
In any case, I suppose that NTR is a matter of perspective, and what I don't see as such, others may consider it that way. For my part I will not continue talking about this.
In my save, the MC is starting a relationship with Mallorie and Ellie (I think I have maximum points with them). With Lucy we'll see, but her attitude towards the MC is quite frustrating.