The first time they danced was on their fourth date.
Alec came over to Magnus' flat, as usual, and found him dancing around the loft, a song blasting out somewhere, so loud that Magnus didn't notice the door opening.
Not that that was a problem. This might only be their fourth official date, but Alec had already come to see Magnus many more than four times, even if he had just popped in to say hi. Magnus was used to it now, and left his door unlocked.
He really had lost track of time, Alec thought from the entryway. Magnus' eyes were closed, and Chairman Meow ran through his feet, the two of them enjoying the music. Magnus was wearing black tight leather pants, a blue t-shirt shimmering with gold thread, and a designer jacket. Alec's eyes lingered for a moment, and then Magnus noticed him and grinned.
"Alec!" Magnus crossed the room and kissed him against the door, and Alec relaxed in his arms. Magnus' hands cupped his face, and slid down his neck, his shoulders and his chest, landing on Alec's waist, under his shirt. Alec held Magnus tight, hands on his back, and suddenly Magnus was pulling Alec away from the door and grinning so much it was getting difficult to kiss him.
"Let's dance", he murmured, and his touch went from caressing to secure on Alec's back, and he held up a hand. Alec felt himself blush.
"I, um, don't know how", he stuttered, hoping he wasn't as red as he felt. Magnus laughed a little.
"Never danced with a partner, Alexander?" His eyes were shining.
"No. I don't think I ever danced, period."
"Well, it's about time we fixed that, then."
Alec wanted to protest, to say that he was absolutely clumsy, that he would step all over Magnus' feet, and that he'd probably cause them to fall over, but Magnus put a finger on his lips before he could say anything.
"Ssh. You'll be fine, Alexander. Just follow me."
Alec followed.
And strangely enough, it worked.
It wasn't perfect, far from it. He did get things wrong, and step on Magnus' feet more than once, and at the end Alec tripped on his own feet and fell on the couch, pulling Magnus down with him, but it didn't matter. But he did follow Magnus, and the music, and they were one and they were dancing, smiling and giggling to each other, and Alec buried his face in Magnus' neck and rested there.
The music played on all evening. They danced, and fell, and got up and danced again, and when Alec went back to the Institute that night, he found himself whistling the songs to himself down the street.
As it turned out, he did like dancing.