No, they don't need males to survive, they need sexual energy in general. There's a whole library conversation about it in Simon's route. Yarra and Qum have each other for that. Before Chapter 1 Yarra was subsisting off a lust amulet, and later we meet Yelarel who survives off lust potions. Yarra had options, but chose to fuck other men.
And it's not explained as a matter of survival, it's part of their "diplomacy" to "further their goals". Which isn't even an excuse by itself - how many NTR plots are there where they make some kind of excuse that their cheating ultimately benefits their partner? Oh, I have to fuck his boss so my husband will get that promotion. Oh, I have to fuck the Yakuza because they're blackmailing us. Happens all the time, definitely still NTR. Even if Simon were present, and ostensibly consented, it'd still be an NTR game with the situation forcing them into it. Even if he fapped in the corner and loved it, it's netorase at best.
Sorry but if the dev didn't want to be branded an NTR dev, she shouldn't have put the girls in a relationship and then had them fuck men outside the relationship, simple as.
Lore-wise, you are correct, it was not a matter of survival and succubi don't strictly need sex to survive.
I don't agree that the game has NTR because NTR is a
fetish built around the act of
cheating, as in betraying the expectations of one's partner and engaging in sexual activities with other people that
should be off-limits.
First, there's literally no NTR fetishization in any of those scenes in the game. None of those scenes have the characters going like "oh my god I can't believe I'm having sex with a man other than Simon, this is so wrong, I can't stop thinking about how hurtful this would be for him and how I shouldn't be doing this" or anything even remotely similar to that. The only 2 moods the writing has in those scenes are "hey I haven't done any succubus seduction stuff in a while, let's see if I still got it" and "fuck I wish Simon was here, sex with him is better than anything else", couldn't be further from an NTR scenario in both of those.
Second, I don't even think you could write a consensual NTR scene here given the way the relationship between Yarra and Simon is in the game. Succubi just add sex to pretty much everything in their lives, the lore makes it clear that, in their societies, as long as you are honest with your partner and don't do anything with the intent of hurting them, sex with others would never be a problem or a betrayal of any kind, so Yarra really doesn't consider those scenes to be in anyway reducing her commitment to Simon. And Simon on the other hand is not an immature dickhead, the guy understands how succubi are and he walks around with a giant harem in which he is frequently prompted to add more women to, he 100% understands it and treats his relationship with the succubi women as an open relationship.
So given all of that, the only way "NTR" could happen with Yarra would be if it was something non-consensual and someone was trying to force her and take her away from Simon, which is literally what the Incubus King of Arclent tries to do with her, and then she overcomes his power and tells him to go fuck himself because she's happier with Simon. The game literally has a scene about shutting down the one scenario where it could feel like an NTR scene, how the hell people still think this was trying to be an NTR fetish section?
But finally, even if you disagree with all of this and just come out of it with the simple neuron activation of "woman I have sex with having sex with other man = I'm upset and she's a deceitful harlot", how are 2 tiny scenes in a 100+ hours long game enough to "brand" the dev as an "NTR dev"? That's just such an overreaction.
