i didn't even think of robert cheating at first i thought magnus was talking about how robert and maryse lied to their kids for years about not being members of the circle rlfnelrnfelnflnerfnlkref
I think it was a reference to all those things tbh waoejfwoa the Lightwoods don’t really have a reputation of being honest about much. In all seriousness, though, it’s very telling that this is the point Magnus chooses to attack – like it’s definitely a harsh blow but it isn’t one that came out of nowhere. Magnus fell for Alec in large part for his honesty. That has always been the consistent thing about Alec – he was always upfront and honest even when his honesty meant he said some foolish things before he could think better of it. Alec has never once lied to Magnus until now.
And honesty is something Magnus values deeply, especially after having been jerked around for decades by Camille but also just in general. You can tell from the friends he keeps, people like Ragnor, Luke, and Raphael who don’t hesitate to tell him what’s on their mind even if they’re not exactly delicate about it. (We haven’t met Catarina on the show yet, but she’s much the same in personality, too, in the books.)
On top of everything else he’s been through and what this means for the Downworld, it’s why Magnus’ reaction is so severe. Like, he was really out for blood and even Luke had to tell him to chill the fuck out. It’s understandable why Magnus lost it, but Luke’s voice of reason isn’t wrong: Alec is trying and objectively, we know he’s a more trustworthy ally than the Seelie Queen. But while it’s true that they’re very powerful, it’s also worth noting that Magnus’ snap decision was to ally with the only Downworlders technically incapable of lying. At its core, this is what it comes down to – that someone he trusted so deeply lied to him.