I think they nixed a sex scene early on for him, once it was decided he'd be a love interest, because they knew where they were going with the character. And, well, Solas withheld a lot of very important things, not the least of which, is that he is basically Dalish Satan, has spies in your organization, gave Cory the Orb, and wants to tear down the Veil - quite probably destroying the world as you know it, and killing countless innocent people in the chaos.
It would have brought up a lot of arguments about consent and power imbalances that the devs probably wanted to dodge if at all possible. Solas' character gets dark enough by the end.
While you technically have more power over him in the game, initially, he is doing things behind the scenes you can't even imagine and have no reason to suspect, until it's much too late.
So yeah, I can see where some players might have issues with how much power he actually has, and what he allowed you to believe, and feel very betrayed, hurt and used, even though Solas is open and adamant that he truly loves a romanced Lavellan.
I get your argument about consent and power imbalances, and I agree. But if I were on the dev team, I'd point out that IRL relationships come and go like a whirlwind, a lot of the time you don't even get a chance to learn all the truths about a person and then suddenly the relationship is over; you actually cannot be entirely honest with your LI's, but I get it: Solas is actively hiding a very, very important part of himself. OK. But does that make their relations a 'violation'? You fell in love with each other for crying out loud, and yes, betrayals do happen. But you did go into the relationship willingly. If I found out such an important secret, I wouldn't immediately jump to the conclusion that they were deliberately trying to trick me or something like that.
This might be a stretch, but I remember reading an article about the devs putting a stop to some 'rape scene' that they didn't realize was that until someone on the team pointed it out - could this have been that scene? Because Solas is Fen'Harel and thus the imbalance between them and his lies by omission are so colossal that players might feel violated if Solas had a normal sex scene?
If so, I feel vaguely insulted. I'm not a porcelain doll, my character chose to be in that relationship and she had a choice to end it, too. I knew he was shifty from the start, and though I didn't know the magnitude of it, that was part of what attracted me to him in the first place. IRL you take responsibility for both your good and bad judgement calls and relationships are a veritable minefield of bad judgements; I had relationships that make me shudder in horror to remember them and I wonder what the hell I was thinking; I'd feel violated if the unnamed individuals would want to press on and rekindle the relationship (pretty much even as an innocent suggestion - yes, it was that bad), but I also know that it was my reckless decision to get into it in the first place and I did go along with it at the time.
I've talked about power imbalances at length in sexual relationships before, including in Solavellan, so I won't touch on that much. Needless to say, at the time of the relationship's occurrence, the romance is designed in such a way to ensure that it is the Inquisitor who is invested, forward, certain, and in control - this isn't something she's been manipulated into, this isn't something she's being pressured into. I think the devs did a good job in portraying that.
In terms of secret identity + consent, I think that's a very legitimate concern and I wouldn't brush that aside. To use a real life example: I read a nasty article today about undercover cops who have had sexual relationships with political activists in the circles they were infiltrating (if you want a link, PM me). Some of these lasted several years, some even resulted in children. If we define consent as something which requires being fully informed, without being coerced or misled, then this arguably doesn't make the mark. Many of the partners involved described at length how heart-shattering the revelations that their partners - who they loved completely - were lying to them, and many of them now struggle to trust other people and form relationships because of those revelations, it's worth emphasising how devestating the effect has been on their lives. I would not be surprised if they labelled what happened sexually as rape in retrospect either.
I should emphasise, this example is
not analogous to Solavellan at all. Nor was it meant to be. A large difference is that unlike these RL cases, where the infiltrators often had family/children of their own and did not care for their partners, Solas does still love Lavellan, his feelings were genuine, and his 'false' identity was rather a different expression of himself, and his words were always genuine. However, what I want to highlight here is that when you're lying about your identity - and yes, lies by omission count here - is that it is questionable how much consent can be 'fully informed'. Even if Lavellan is taking an active role, of her own initiative, without coercion or persuasion, enthusiastically, it's questionable.
'questionable' or 'dubious' isn't something that should exist when it comes to sexual consent. Certainly not within a fictional depiction of a healthy relationship, as much as anything else.
I am glad, thinking on it, they did not include a sex scene.
Plot armor doesn't bother me, and neither do plot rails for that matter, as long as they make sense, and IMO Solas's do. I don't care how much of a genius special snowflake you headcanon quizzy to be, no reasonable person would ever have suspected all the crap he's up to while he's with the Inquisition. And by the time you do get the truth, you're dying from the Anchor and he's extremely powerful. I'm ok with quizzy being helpless in that situation.
In contrast, DA2's Anders plot armor drove me nuts. Because there is no reason for it, other than "THE PLOT DICTATES."
Fine, let the plot dictate, but at least try to explain it away. Anders all but tells you he's up to no good. But because THE PLOT DICTATES, the authority figures who are happy to make mages Tranquil for sneezing wrong don't follow up when the freaking Champion of Kirkwall tells them Anders is up to something? You can't even just murder-knife him yourself and have it done with. And I say this as someone who sided with Anders in my canon run 
Something is very wrong when you can sell Fenris back into slavery for no good reason, and can't arrest Anders for legitimate reasons 
They definitely missed a trick here.
I know there are plot reasons for portraying Anders as quite isolated - I think that contributes to his worsening condition, a lack of support. However, something which irked me was that Anders was supposed to be quite active in the Mage Underground, a network of apostates which we never actually saw or felt any influence of asides from his personal quest (and we don't actually see it, Anders just talks about it).
I think if a pro-Templar Hawke sells Anders out, I'd like to see him leave the party, perhaps get captured/be given to the Templars by Hawke, later escape with the help of the Underground, and then continue on to complete the chantry explosion with more determination and fervour. Even better, as a consequence, if you kicked Anders out of the party, you have no opportunity to kill him, because as it is in the final scene Anders essentially offers his life up to you as he is utterly repulsed by his own actions, by himself and his capacity to cause harm in general, and sees Hawke as the moral authority here. If you gave him to the templars, Anders would have no respect for your opinion whatsoever - so why would he do such a thing? It'd make an interesting ironic twist, it'd keep the DA2 timeline in tact, and perhaps be a good way of introducing other apostates which had a hand in the action.