Mistake

"Do you, Alexander Gideon Lightwood, take Magnus Bane to be your lawfully wedded husband?" Brother Zachariah asks.

"... There's been a mistake," Alec says. The attendees gasp in shock, some covering their mouths with their hands.

Magnus looks at him. "Alexander..."

Alec ignores him. "This is a mistake," he says, shaking his head.

OR

Malec (crack-ish) wedding based on a Tumblr textpost