Those who don't want trans players to play against girls cite a parade of horribles: a girl might might see a boy's genitals in the locker room; there is an unfair "testosterone-based" advantage; a boy might fake his gender identity so he could be a superstar on a girls' team; a girl might get scraped by beard stubble .... etc ....
These are all hypothetical outrages. How many actually happened? Well, the best way to find out would be to look at a State which has allowed trans athletes to play with and against players of their gender identity. Then count up all the actual outrages.
California has allowed such competition since 2015. The right extends to all age groups in elementary and secondary schools. The right extends to using bathrooms, locker rooms, and any other facilites. It is part of the California Education Code. In pertinent part, CEC Sec. 225.1(f) states: (f) A pupil shall be permitted to participate in sex-segregated school programs and activities, including athletic teams and competitions, and use facilities consistent with his or her gender identity, irrespective of the gender listed on the pupil’s records.
Maybe some of the anti-trans posters list all of the horrible things that actually occurred in California as a result of allowing trans-athletes to complete.