Jace knew that his parabatai thought that he was in love with Magnus. He wasn't sure, only because Alec had never expressed any interest in dating anyone—male or female—before he met Magnus. Then suddenly he was head over heels for the warlock or the older man had put some sort of spell or curse on him to either trick him or make him feel things that weren't completely genuine.
But then he saw the way that Alec smiled at Magnus—truly smiled—the kind of smile that he'd never seen him use before, it was the type of smile that lit up his entire face. It shocked Jace just to see how happy Alec looked, and all because the warlock just walked into the room.
Now Jace was making his way into the last training room. Alec never used to use that room, that was a relatively new development. That training room was the only one without runes or any other type of angelic marks, which meant it was perfectly safe for Magnus to walk in or out of. He'd thought that Alec was alone in that room.
He walked into the room without knocking, after all it was a training room. Ideally this meant that people were supposed to be training- not sticking his tongue down his boyfriend's throat, clutching at the warlock's clothes like if he let go the warlock would disappear. He was about to clear his throat; to try and get his parabatai's attention when Magnus walked them to the nearest wall, pushing Alec against it, cupping the niphilim's face, kissing softly despite the fierceness of their hands.
Jace couldn't help but smile with the way that Alec's hands gripped Magnus' hips and the way he melted into the older man's touch. The Alec that he knew was not the Alec he grew up with—this wasn't a very bad thing. To Jace it seemed like Alec didn't start living until Magnus came into his life.
He was about to turn around when he felt a hand on his shoulder. He turned to see Robert looking at the pair. There was a hurt in his eyes that Jace couldn't quite place.
The warlock and the shadow hunter seemed oblivious to their audience as Magnus gripped Alec by his hips, they stopped kissing long enough to look into each other's eyes; they seemed to reach a silent agreement as Alec braced himself against the wall. In a smooth flourished of his arms, he gripped Alec's waist to lift him off the ground, the younger man wrapped his legs around the others waist, while his hands wound themselves in his hair.
Robert grabbed Jace's arm, getting the other man to turn his back on the private scene. "Come Jace—I'm not sober enough for this."
All Jace did was smile. Robert was trying and that was the best and only thing that Jace could ask of the man.