Gale is a human colored by a disdain for his humanity. He goes into detail multiple times about how his humanity is a dissatisfaction. He wants to go further into the Weave, to know divinity completely.
Mystra is the Weave. She is magic, full stop. This is explicitly stated.
But when Gale held her, kissed her, there was nothing there to touch. Skin more like marble in its flawlessness. Eyes more like crystal in their luminance. She always smelled of sharp rosewater, always tasted of mild sweetness. Never a hint of sweat, of saliva, of any of the unclear oils and bacterias that infest the mortal form.
Somehow, I can't imagine Mystra ever laid with him as a man. Why would she bring herself low, to get in a human's actual bed? What would even be the point, when an ocean of cosmic perfection lay above?
Did it disgust him, then, to travel so high, lay with the very form of Magic, only to return to his meager human body? To be the owner of skin that smelled of sweat, skin that had scars and wrinkles and marks, skin that aged?
When Mystra cast his aside for his folly, did he long for the divine from the confines of his humanity? Did he hate the needs and limits of his body, the various hungers that cried out to be sated, when all his mind craved was that glittering perfection?
Mystra loved Gale's magic. Mystra loved him as a tiny expression of her own self.
Gale is his mind, certainly. His mind is where the magic is. His mind is remarkable. His mind is worthy of love. But this human thing, this temporary human thing that is a body, is not him in the same way. It's needy, and he wants so badly to escape from it into the divine.
And maybe that's what surprises him so much about being loved by a mortal. That someone would lay with him, Gale the man. Would bury their nose into his neck and breathe deeply the scent of him, would kiss the crows feet at his eyes and relish in the taste of his mouth.
Gale knows the touch of perfection, but the embrace of humanity has a deeper flavor, one that grounds him to the earth. One that makes him realize that flawless, perfect love is a distant thing, an inhumane thing.
