Channeling the libfem I used to be in my 20s (way before trans issues took off - and I hasten to add as a sporty woman, I was never so daft as to think there was no physiological difference...) I think the reluctance to "see" physical differences comes from three main factors.
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An over-emphasis on the importance of socialisation relative to physiology.
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Cognitive dissonance -"if I admit to myself how much stronger men are, and that some of them are bad guys, I'll never leave the house again... so I'll pretend that Black Widow, Wonder Woman, etc. etc. are realistic, and that with the right jujitsu moves I too could fight off a rapist."
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A worry that if we admit to the existence of physical differences in sport, that will function as the thin end of the wedge, and we'll end up having to admit to cognitive differences as well. (I don't think this fear is well-founded - insofar as any studies do show cognitive differences, the d-values are tiny, way, way smaller than d-values for physical differences; and the studies are rarely robust across different methodologies; and you can't even in principle filter out the effect of nurture and brain plasticity).
Re. 1, the importance of socialisation (and apologies - this really is a TL:DR): I can see how this happens because in some sports women are still playing catch up. Take football AS AN EXAMPLE - a sport I used to play. I started aged 30-something (friend talked me into giving it a go, I fell in love with it). By about age 7 or 8 DS was more skilled than me, because he had played for hours and hours, obsessively, from about the age of 2. (Although it's been debunked as being the whole story, Malcolm Gladwell's 10,000 hours to excel at something rule has some merit).
Now look at your average upper primary school. You have 4 classes of 30 - so 60 boys, 60 girls. At least half of those boys will be obsessive about football. Maybe 1 girl out of 60 will be. Simply because of social pressures. There will be sporty girls - doing gymnastics, or swimming, or... But not football. So you immediately have a massively larger pool of talent. This is still the case today.
So a libfem can point to this socialisation and say "that explains the difference..." (In fact there used to be a poster on here who used to try to push this position).
My view is that if women's football was taken as seriously as, say, women's tennis (if we'd never had the FA ban on women playing on FA pitches, which ran from the 1920s to the 1970s!) we'd see a lot higher standard of women's football. It's really turned round out of all recognition in the 20 years I've been following the game - standards have rocketed. But just as Serena Williams couldn't take on a man in the top 100, women's teams will never be able to compete with the sheer speed, strength and physicality of men's teams.
Socialisation is part of the story, but it's never the whole story.