I agree fully with what both of you are saying in the sense that:
A) A netorare story relies on the protagonist ultimately never fully catching on until it's too late, otherwise they're seen as pathetic or incompetent- Or both.
B) Leto has no reason to dislike/hate the antagonists- AND that's an issue.
It's obviously been written that Leto is oblivious, if you can even call it that. It's logical sense that he wouldn't know this grand conspiracy happening behind the curtains. Why would he know? Haywire_hero is correct in this sense. My issue, and this might be premature considering we haven't truly hit the climax of the story yet, is that having conflict between the corruptor and the protagonist is important to the netorare fetish.
The logical endpoint of NTR is that the corruption gets to a level where the girl actively chooses to discard the protagonist. Right now, for all of these women bar Rose, there is no choice to make. They're not having sex with Leto, they aren't in a romantic relationship. Haylen has her hang-ups, but Lily and Cyanna are single women who can make their own decisions. In fact, it's reasonable to suggest that it's good for Cyanna to be with Ulrod instead of pining over a man in a long-term relationship.
Leto is a good person, an upstanding guy. Why does he care about Cyanna sleeping with Ulrod? Why does he care about Nezar training Lily, or Jason and Haylen?
Some of you are probably thinking: "The NTR comes from the loss of their relationship."
You are right, that is a form of netorare and that's fine... But doesn't it come across as immature for Leto to get upset that Cyanna, for example, is in a relationship? I don't think any of us consider ourselves an NTR protagonist when one of our friends gets in a new relationship and has an abundant sex life. Hell, I've seen a fair few friends get changed for the worse by their partner. For a mature and normal person, this doesn't really cause much heartbreak.
So we need some form of betrayal. Something that goes from, "You're my friend, you can do what you want" into "How could you do this to me?"
With Rose, it's obvious. I don't necessarily need Leto to hate Nezar, Ulrod, or Jason. I just need the twist of the dagger. Cyanna could end up complicit in the conspiracy at some point- I can see a scene where she's commanded to guard something and ends up fighting and defeating Leto in a duel. Her choosing to protect a grand conspiracy out of lust for Ulrod's cock, turning 'evil' would be a good example.
Lily and Haylen are a bit more complicated as they aren't involved in the main story (why the creator has chosen this, I'll never understand. Especially considering everybody plays the side stories anyways!).
Sorry for posting so many long comments but just wanted to emphasise my point on why I think Leto should hate the antagonists. There's still plenty of time for that hate/dislike to develop, too. I'm sure considering where the last main quest left off, Leto is going to discover the conspiracy finally... Which in turn might aid some of my issues.
Tldr; Aside from Rose, none of the characters feel as if they're being stolen from Leto and there isn't any betrayal going on.