I think it is brainwashing.
Going from Cait type sexual personality and relationship with the champion to absolutely obsessing with us and only caring about us is such a huge departure I'm terms of character appeal.
For if Brint always had this subby/monogamous personality inside of him, it sure does not really show up in his own content where he is a manwhore and games self proclaimed stud. You can say that armor brought out Brienne out of him but the way it is presented to me still sounds/looks like brainwashing that goes beyond slight influence. You can argue how much armor did change, but it is undoubtedly some kind of magic curse.
Now, let's say hypothetically that the game gave us option down the line to influence both Brint and Brienne to take on each other's traits, than sure, I can see argument that armor just influenced him some what. But as it is, Brint and Brienne are different companions in terms of appeal, content and characterizations.Which is why, from meta perspective, I think Wsan did it. Monogamous, subby NPCs are way more popular than open relationship ones.
And I, like many others, am all for it. For I can not see me getting attached to Brint with tits as I did with Brienne.
Now, Brienne as a guy tho? Now that is another story.