The breakup starts with Magnus. Well, no, it doesn’t. The breakup starts when Alec finally says the words “I’m gay,” and Jace says “Okay?” and Alec looks at Magnus, out and proud, and happy, and he knows that he wants that. He wants to be able to hold hands, and kiss, and have sex without worrying about leaving hickeys where they might be seen. Or: Alec wants a public relationship. He can't do that with Jace, so they break up, not very cleanly. Magnus steps in, and then Jace gets jealous.