I should be more passionate about Bull's loyalty to the Qun despite its treatment of Saarebas, but the truth it I already have a long laundry list of complaints against Bull, the Chantry, the Circles, the Templars, lack of meaningful responses to lots of things many of my previous characters had complaints against, etc. to really care by the time it got to the question of a mage Inquisitor befriending or romancing him or not.
If it really bothers you that much, write a fanfic where your mage character lets him have it for his views. Or just don't recruit/befriend/romance him.
I see the Vashoth option in dialogue. I think oh, yeah..cool. Oh, Vivienne practically snarls at me saying that the Vashoth cut out the tongues of their mages and even outright kill the mages they think may be too weak to master their magic. I didn't know this. Vivienne ends it by saying...and snarling at me, that, "So you [Vashoth] are no better than all the rest."
I was surprised by this coming from Vivienne. The anger about how mages were treated by the Vashoth.
Then, "I've taken up enough of your time, darling. Don't let me keep you."
This is another instance of the game not being friendly to newcomers, or not explaining its lore very well.
Codex entries to previous games have revealed that Tal'Vashoth don't really know how to form communities since they were told how to live by the Qun for so long. As such, they don't really know how to train or deal with their mages. Also, despite outwardly breaking away from the Qun, they're still subconsciously swayed by it. One of the ways this manifests is how they view mages. Despite breaking away from the Qun, many still have that deeply ingrained prejudice and distrust against mages, so many Tal'Vashoth will cut off the tongues of or kill mages they don't trust, just like they did in the Qun.
I think that's what she was referring to, but if one isn't a complete lore nut and hunted down every piece of ambient dialogue, party banter, or codexes from previous games, one wouldn't know what she was saying. I knew what she was talking about, but that's because I'm obsessed and read every scrap of lore I can get my hands on.
The Vashoth situation is not helped by how we've never actually encountered vashoth before, so we have no idea what their lives are like. We've seen humans, Circle mages, Dalish, dwarves, etc. live their daily lives each game, but we've only seen stranded Qunari (Sten and the Arishok) or Tal'Vashoth canon fodder (DA2), so it's hard imagining how a grey giant would live their daily lives in their own communities.
Less scrupulous Templars could always accuse their ex-lovers of being abominations or blood mages and kill them on the spot too, maybe even order their tranquilization. Either way, Mage/Templar relationships are bad and creepy.
Paraphrasing Red from Orange is the New Black, "there's no such thing as consensual [with a guard] in a prison." Mages are prisoners and Templars are their jailers. That, itself, involves a significant power-imbalance that cannot be ignored. The Templars have all the power and the mages have no power. Now, that doesn't mean the Templars will abuse their power, but the power still there, and it changes things.
I agree with you that I cannot imagine Cullen with a mage (especially an elven mage, who are double-hit by the Chantry's demonization and opression of both elves and mages) without it feeling icky or unballanced to me. Despite all the atrocities Meredith committed he still believes in the Circles and the Templars. He still believes mages should be locked up and monitored by Templars. I can't romance him as a mage (especially Dalish mage) who values their own freedom because you're with a man who fundamentally believes that you alone should have your rights and freedoms stripped away, because you're a mage and he's not.
Bull isn't any better, but then I don't want to romance him at all, let alone as a mage. (Totally admit to personal feelings here.) But it's kind of different for me because if you can convince Bull to turn Tal'Vashoth. You can probably headcanon that he eventually gets over his blind loyalty to the Qun enough to recognize that how they treat Saarebas is wreched (same with their lobotomy gas and whatnot) and there are other ways to teach mages that doesn't require mouth stitches or control rods.
I guess Cullen kind of bothers me more because you can never really change his mind. He never really stops thinking the Chantry, Templars, and Circles are the best way to deal with mages. I mean, sure, he's become open-minded enough since DAO and DA2 that he doesn't object too strongly to you allying with mages, but he'd still prefer the Templars and Circles and you can never really call him out or change his mind on it.