Their premises are, TWAW, and that it is impossible for recognising the "rights" of someone to be unfair for others. Rights in this case meaning right to identify as a woman.
I think this is right @Goosefoot.
You start from the premise TWAW, you arrive at the conclusion "therefore they should be allowed to play women's rugby and anything else is discriminatory."
Then you look at a photo of the player for Porth Women's team who "folds opponents like a deckchair."
At this point, formal logic allows you two options.
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You think "That's crazy", treat it as a reductio ad absurdum of your starting premise and realise that TWANW.
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You hang onto your premise, deny the evidence of science and your own eyes, and insist that women's sport be shat on from a great height.
Unfortunately, human nature and lack of critical reasoning skills being what it is, a lot of well-meaning people in the "be kind" camp instead choose to live with an untenable, illogical third option of "be kind, parrot stuff about inclusion, and do not under any circumstances allow yourself to look to hard at the consequences." Cognitive dissonance writ large.
Any sane person should be in camp (1), there's nothing we can do about camp (2), but the really worrying people are those in the untenable third place getting splinters up their arse, because they're the people who'll vote Labour even when Lisa Nandy says she's cool with male rapists being placed in female prisons, because they want to "be kind."
(2) is really an extreme position, the real political fight is between the people who instinctively think "reductio ad absurdum" and the people who go "la la la, not listening" and continue to act as unwitting supporters of position (2).