Sending her to Aron for NTS actions will reduce her fidelity. However, there's always follow-up sex with Elisabeth in where a bit of fidelity is restored, so it can be grindy. Just don't do any "Love" actions because that wil raise fidelity even more.
But yeah, what Hefiron said, there should be changes in the future with those events.
Losing battles with certain NPCs in the story/the one side quest in the game will also cause an NTR event and you'll lose a ton of fidelity that way as well, about 10 points each I think.
And doing any "evil" actions or choices will reduce her fidelity, at least for now. There is an "evil" path that will eventually be in the game (there's even one major side event in the game right now that you can do to corrupt yourself with some evil goddess or something), and IIRC, the dev said you can turn Elisabeth evil as well in the future. So I would imagine if you make Elisabeth evil, then your fidelity probably goes up doing evil actions, and down if you do "good" actions.
I didn't actually play this yet, If the dev is thinking of reworking the mechanics, I'd like to offer an advice.
The problem with the current system seems to be that fidelity isn't set up to be a great path selecting variable. If sending her on NTS events is what lowers fidelity, and that will eventually lead to NTR, what about the people that enjoys NTS but not NTR? What about the people that enjoy NTR but not NTS? And if doing evil acts decreases her fidelity, what about the people that want a evil dom MC taking his harem?
From a story or character perspective, perhaps the current mechanic could make sense, but it doesn't lead to a clean route selection system.
There are various ways you might address this, but I will just offer one simple option. Gives choices for the female protagonist to determine her own fidelity. Even if you don't make her a full on dual MC, the fidelity can be a choice from her side so that even if the MC does all the "right" things to not pass her around, she may be tempted and cheat on him if the player makes her. Or the MC might do some evil acts that normally pushes her away, but the player can let her justify his actions and stay loyal to him, eventually maybe getting corrupted together with him as he forms his own harem or something. It's just much easier to plan route selection this way.
If giving FMC choices in unacceptable, I guess you can try to get creative. I'd just point out two considerations when it comes to player psychology. Unlike NTS fans, many NTR fans don't want the MC to have a say on the matter. Part of the NTR narrative is that it is happening behind his back or against his will. So how do you create this scenario while still giving the
player a route making decision? That's what you have to figure out. And for the evil MC, most fans of an evil MC probably don't want him to get punished by getting cucked. It's more of a power fantasy angle of enjoyment, so decreasing fidelity of the girl based on his evil actions could be contrary to what players making evil actions would want.
Edit: Ah, I just read that last paragraph about turning the LI evil. That seems cool. Anyways, I wish the dev luck. The many routes seem very ambitious.