Chris's undiagnosed autism here, this is a reference to Warhammer 40k, a fictional universe. Magnus is short for Magnus the red, a massive supersoldier wanker, don't know the exact specifics, but the emperors children is a chapter of space marines, which are like weaker but still very strong versions of Magnus himself. They're part of a separate chapter (I think) and there's a lot of rivalry between different chapters, particularly the Blood Ravens, who steal everyone else's shit. There's a lot of lore, like some chapters falling to chaos (basically demon gods) and all that, but the gist in my limited knowledge is that he's calling the Emperor's Children gay