It's... sort of complicated.
NTR is more about the betrayal of trust than the physical act of bedding someone else. In Adorevia, people in your party are very sexually liberal for the most part - they're far more comfortable with casual physical flings than in the real world. As far as I can tell, the love interests you can form strong emotional bonds with (as opposed to, say, the group tentacle monster or the succubus) won't ever fuck someone behind your back because of your inadequacy or as a means to belittle the PC.
However, it is possible to see someone you're involved with having sex with someone else, especially if you've pushed them to be more sexual or corrupt. Even so, it's not treated as "you're not good enough" but rather "here's a chance to have a threesome."
I can't definitively answer your question because I don't typically play this kind of game in a way where my PC would be bothered by it - but I will say that in both my male and female "do good and fuck everyone" approaches, I have never had someone in the party run off and betray me. Did I walk in on the woman I was having a kid with domming a tentacle monster? Yes. Did she know I was watching and then invite me to join her? Also yes. It's that kind of game.
