Oh! Yes, very reasonable.
Okay so, one reason why deception is bad in intimate relationships is the obvious surface-level reason of "if they knew you were deceiving them, they would be sad", and that can be a sufficient reason if you care intrinsically about that, but it's just a starting point. There's plenty more to be said on the subject.
Deception creates distance, for one thing. One of the purposes of an intimate relationship is intimacy, closeness, sharing yourself with another person and being shared with in return. It can feel awkward or uncomfortable to try to be close to someone while keeping a secret from them that they would be sad to learn you had kept. It can be unpleasant to try to be close to someone who bases part of their opinion of you on false beliefs, when they tell you what they think of you and you know they only think that because you lied to them. When you're with them, you have to spend thought and energy on tracking your secret and making sure you don't accidentally reveal it; you can't be fully and casually open with them, can't speak carelessly in front of them. People often pick up on that kind of distance and, even if they don't have any idea why, notice that their friend or partner is acting oddly towards them.
Deception also makes relationships fragile. If you're keeping a secret from someone, you have to continually put work in to maintain that secret, and it might take only a few slips of the tongue before your deception is irrevocably demolished; also, other people who know your secret might reveal it, either accidentally because they didn't know it was a secret, accidentally because they weren't as careful as you, or even on purpose, whether because they want to damage your relationship with that person or because they want that person to know the truth or for some other reason. In fact, even someone who doesn't know your secret can inadvertently reveal it by revealing evidence for it that they themselves did not realize had those implications! Things often don't go well from there, however it came to that point. By contrast, if you're keeping something from someone and then you decide to tell them yourself because you want to stop lying to them, especially if you're doing it out of consideration for them or for your relationship, they'll probably feel more favourably towards you than if they found out by accident. They'll tend to be less upset and more open to reconciliation.
And deception damages trust. The more you lie to people who think they can trust you, the less it is the case, in practical terms, that you are trustworthy. That's obviously bad in an ethical sense, but it's also bad in a practical one? Trust is a tool for coordination; it lets people cooperate with one another toward shared goals without spending effort and time and energy searching for potential betrayal. When you give the false appearance of being trustworthy, you're contributing to a state of affairs where that tool is less useful, where people benefit less from using it, especially around you in particular, because you will abuse it to betray them. It's some of the same problem as Tanthe was complaining about with destroying people at the heart of the resistance against the Red Queen specifically, though much less immediate, more diffuse and abstract: there's an important project happening, one that can benefit you as well as the people working directly on it, and you're sabotaging it for your own short-term gain. (For the personal benefits of being able to rely on genuine trust, Aire need look no further than the reasons she herself has been trying to be actually-trustworthy toward Tanthe. Which Tanthe really appreciates! It's good to know that that's a thing Aire can do, and good to know that she's putting in the effort to do it!) Under this framework, manipulating people into trusting you so that you can use that trust for purposes they would disagree with is generally bad, but it's especially bad to do in the context of an intimate relationship, where people normally have the most reason to trust each other and are relying on that trust the most.
She could probably come up with more reasons if she tried, but those are the big ones, in her opinion.