i think acknowledging freya, and merlin's love for her, and particularly the fact that he was fully prepared to run away with her represents a capital P problem for merthur shippers, in the sense that (shipping goggles on) freylin just doesn't make sense: it's a disruptive, destabilizing element in the narrative
when you think about how ready merlin was to leave everything behind (coff coff arthur) the entire architecture of fanon merthur starts to collapse: the 'standard' reading that by this point they would and will sacrifice everything for each other, is fundamentally incompatible with freylin
people in the tags are saying that it's because making merlin gay would solve a lot of headaches for the average merthur shipper, and although i agree with the sentiment that the dislike for freylin is dependent on its interference with the main m/m ship, i think there's more to it than just that
freya is, i think, a particular inconvinient character for merthur shippers. compared to other merlin ships, that can easily coexist with merthur to varying degrees, freylin as it's presented in the show simply cannot. this ship creates a fracture in the fanon view that is impossible to ignore: when push comes to shove, merlin does not choose arthur
there is no ambiguity, no room to keep things open to interpretations: merlin decides, without a second though, that he's willing to leave camelot and arthur and his destiny behind for this girl "he barely knows", and it's not just empty talk: he is 100% ready to do it (he leaves camelot in 1x10 too, but for his own mother, and there are no conflicting romatic feelings involved in the decision)
it's the definiteness of it, of merlin's choice, that clashes with the way merthur thrives in the blurry, unresolved spaces of every other relationship. even arwen it's not that conclusive, to me, in the sense that it never categorically denies merthur, and even if you don't want to straight-up ignore it, it can still easily become subsidiary to merthur (see: comphet arthur readings, poly interpretations, etc.). i always wonder if people's response to freya would be different, were freylin not so 'final' -- if merlin was shown to love freya with all he had, but rejected the idea of leaving arthur's side
in fact! the second reason i think freylin is particularly hard to digest (though i admit this might be a bit of a stretch), it's because it frames merlin in a way that merthur viewers cannot accept -- it's him who's supposed to be aware of his feelings for arthur. it's merlin who's entrusted with the knowledge of what they truly mean to each other. it's merlin that's been told, again and again, they are two sides of the same coin. arthur is granted much more leeway, because he's terminally oblivious, in canon and fanon alike -- but never merlin. merlin knows. when merlin chooses to leave arthur for freya, it's a betrayal of the trust the viewer has palced into him to validate their favourite ship. the fact that merlin, in that moment, does not prioritize arthur at all, creates a kind of cognitive dissonance that is very hard to reconcile
plus, merlin acts selfishly in a way that the viewer simply won't allow him to: he's the one who's expected to renounce everything for arthur, for their love and the destiny they share. this is the version of merlin that merthur fandom generally gravitates towards: a merlin who is defined by his 'martyrdom', and whose sacrifices are always justifiable within the framework of an all-consuming devotion. by loving freya and choosing her, he also chooses a version of himself that the (merthur) audience never really agreed to, and doesn't want to recognize