Ah, thank you for explaining that.
I do understand how it's annoying. The trans subject is very complicated and cannot really be summarized in a soundbite, which always makes people on both sides (at least online) seem fanatical about it.
The case-by-case stance has merit and is one I would definitely consider exploring further. On the face of it I would still note that even after sex-reassignment surgery the bodies of the trans-woman retain those dense bone structures and fast-switch muscle fiber levels that continue to grant them enhanced physical performance levels compared to their "normal" female counterparts. Therefore they are effectively using a male body in field of female competition, which is sort of like one athlete in track and field taking a ton of steroids and performance enhancers before a race and still claiming they don't have an advantage over the others.
Now, this is a really basic overview of my position and to really delve into it would require more time and effort than this website allows. Take for example the 1 in 50k or so people who are born with chromosome sets that mean they are actually neither male or female...how do they know which side of a sport they should play in? Or for a trans man, who would be carrying a woman's body into a competitive field loaded with male bodies, should they be allowed to take steroids to boost their performance to a potentially equitable level? Would that give them an unfair advantage ?
It is a lot more simple and equitable to say to a trans person, "Listen, I know you don't feel like you belong in this field (men's/women's [insert sport here]), but on the only level that matters here, physical performance, you are still a [man/woman] despite the best science can do. Therefore you would have an unfair advantage or handicap if you competed outside the field of your physical equivalents."
Again though, that is just how I think we should operate moving forward. I understand that it is not a popular view, but I think it is the best one.