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It all started with a school reunion.
Although some might say it started way before, when Magnus first laid eyes on Alexander Lightwood and they had their first argument 5 minutes later. Alec was one of those insufferable people who used sarcasm very intentionally, to drive others out of their skin and get in the way of Magnus's usually very welcoming and friendly attitude.
They fought consistently until they graduated, about anything and everything. Their views were clashing mercilessly at any given opportunity, and there wasn't much slack either of them was willing to give the other.
True, there was this one incident where Magnus pretended to be Alec's boyfriend for a few days. And there was this other incident where Alec saved Magnus's cat very heroically.
But other than that- yes, they totally disliked each other.
Magnus never expected from Alec to give him any attention during the reunion.
And the proposal he received, he expected least of all.
''We've known each other for a very long time Magnus. It's no secret that we've never seen eye to eye, but the heart wants what the heart wants. We matched. I'm afraid there is only one thing left to do,'' he didn't get on one knee, but he did come a few steps closer. ''Would you be so inclined to tie yourself to me and be my husband until you get completely exasperated from it?''
''NO!''
''He's still in denial,'' Alec explained to their surprised audience.
Magnus's mind started working in overdrive.
Alec was doing this because he was tired of listening to Sebastian talk in length about his recent marriage. Sebastian was Alec's ex boyfriend from back in the day, and most of the evening he spent raising toasts and feeding Alec lines like: 'It'll happen someday for you too, I'm sure of it. You're still young, don't give up. Not all can find their soulmate as easy as I.'
That was why Alec was doing this. And because Sebastian's many toasts hadn't complied with the fact that Alec was a lightweight in a fit of sarcasm.
Magnus grabbed him by the shirt, excusing himself and rushing them both outside.
''What do you think you're doing?'' he demanded to know.
''It just occurred to me,'' Alec said shrugging. He must have had more to drink than Magnus feared.
''How could such a thing occur to you? What did Sebastian say?''
''When no one was listening... he implied I was a rubbish kisser. And I thought... you pretended to be my boyfriend once-''
''Maybe I could pretend to marry you as well?'' Magnus snapped.
''Sorry,'' Alec said finally, looking sincere.
Magnus gave him a critical look at first, but he also carried him home that night and stayed on the couch because Alec asked him to.
Their story started after that reunion and there were other incidents after it. All minor insignificant occurrences of course. Because it was well known that Magnus and Alec disliked each other. Until they didn't.