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Trans Girls are Girls, and Trans Women are Women. Period.
Even Caitlyn Jenner.
Former Olympic athlete Caitlyn Jenner recently announced her candidacy in the election to recall California Governor Gavin Newsom. Unsurprisingly given her Republican politics, she is now catering to the fearmongering about trans girls competing in school sports alongside their cisgender classmates.
Caitlyn Jenner is as representative of trans people as Ben Carson is of Black people or Milo Yiannopoulos is of gay people; i.e., not at all. She got her victories and medals in her youth, and is now throwing all trans girls who would also like to compete under the bus, misgendering them as “biological boys” in the process.

Trans girls are real girls, and trans women — Caitlyn Jenner included — are real women. They are not “biological boys” unfairly competing with “real” girls. Claims and concerns about physical advantages over cisgender athletes have been addressed, discussed, and debunked, ad nauseum.
Saying that you support trans people but draw the line at them competing alongside cis people is no different from saying that you support lesbian and gay people but draw the line at same-sex marriage, or that you support Black people but draw the line at Black Lives Matter. And yes, I am all three of these things (trans, Black, and in a same-sex marriage) so I have every right to make that comparison.
I have no interest in debating this. I want actual allies from the cis community to stop the relentless attacks on trans people. If you need some good background material for your arguments, I recommend following Julia Serano and Cary Gabriel Costello.