We do not prevent cisgender girls from joining girls' sports teams based on height, weight, level of past training, muscle mass percentage, or any other "biological characteristics". Chromosomes and testosterone levels are widely varied among people assigned female at birth, as well, and there are no reputable studies showing that higher levels of testosterone or an XY chromosome automatically correlates to success in sports. Rejecting transgender girls from joining girls' sports teams is based on junk science and is a ridiculous attempt to further marginalize girls who simply want to participate in the same sports that cisgender girls do.

In terms of behavior - anyone, cisgender or transgender, can bully, harass, and sexually assault anyone. Codifying discrimination against transgender girls won't make cisgender girls any safer. Promoting the sexual assault and harassment of any girl who wants to play sports by mandating 'genital checks' would align with a culture of abuse, and does not protect anyone.