I think it's fair to say that Gale's relationship with Mystra isn't meant to be analogous to child abuse in our real world, but it is meant to (nonetheless) be an incredibly toxic and dysfunctional relationship that is arguably still a flavor of abusive if not outright abuse itself.

Gale was (presumably) an adult when he began his sexual relationship with Mystra, but:
1) she was his goddess
2) he had known her since he was underage (so "grooming" allegations can still apply in a manner of speaking, although given the way gods work in the Forgotten Realms it is complicated)

That layers on some definite "ick" no matter how you slice it, because the power imbalance is incomprehensible. Even if Gale "pursued" Mystra, she had every scrap of power, authority, and the position to tell Gale "no," and, quite frankly, should've known better.

Now, Gale did overstep Mystra's boundaries when he sought the Netherese orb, and that is where a lot of criticism of Gale comes into play (and it was an egotistical and self-serving act under the surface), BUT--
1) Mystra never once told him what it was (and Gale never even learns about its nature until Act 3 of the game); yeah, what he did was dangerous, but he had no clue what he was doing. It was foolish, but not malevolent.
2) Mystra had the power to deal with the orb at any time (as she is capable of removing it after the events of the game)
3) Gale only sought the orb once Mystra had shown Gale beyond the veil and told him he could never have it (again a case of "Mystra should've known better than to taunt a wizard with forbidden and unreachable magics")

This means that while, yeah, Mystra could've smited Gale or whatever for disobeying her, she instead left him to suffer with this agonizing, debilitating condition without any possible solution to it, until she later sends Elminster (a grandfather figure to Gale) to tell him to kill himself. This when she could've cured him, given him a heads up, explained what happened, but she never tells him anything. Just leaves him in a lurch.

I do tend to interpret this as "a test" of sorts that Mystra was giving Gale: to see if he would turn out to be another Karsus or whether he just made a mistake. And, depending on the events of the game, those are the two possible roads: either it is that Gale made a mistake, grows from it, or Gale succumbs to his worse traits and overreaches further to become a god.

But a person's romantic partner shouldn't put them through some trial-and-error, twisted mind games like that. That's where their dynamic just becomes gross to me. It is one thing if Mystra remained Gale's cold and aloof goddess with whom he had a strained relationship, instead there's the element of that was his lover too. And I think that goes to explain a lot of why Gale has such an inferiority complex simmering under his blustering ego. He wants to be equal to his partner, knowing he never can. It was never going to work out and was just an exercise in torment.