The groan is a victory. Magnus isn't a particularly closed off person in general but there are parts of him that he hides. Alec isn't so foolish as to think he has everything of his boyfriend even now, but he has the willingness to have everything of his boyfriend and that makes the difference.

He smiles, even though Magnus won't see it, and leans in to drop a kiss at the base of his neck. "Oh, so you do have steel muscles, and you're just hiding them because it's easy. I see how it is." There's nothing but fondness underlying the tease though and he slowly works his way down from Magnus' shoulders to the middle of his back. "I'm going to remember that the next time I decide to carry you."