Occam's razor:
when you have two competing theories that explain something equally well, the simpler explanation—the one requiring the fewest assumptions—is usually the best choice
Fuck man, these theories don't even explain anything equally well. We could just use diet Occam's razor, Occam's razor light, or low-impact Occam's razor for this one. You said it plainly and simply up there: boys have a penis, girls a vagina.
Oh but but but what about the trans boy who got on puberty blockers at age 9, never went through puberty, and isn't producing T at the level of a biological male? Tough shit. You made a decision for your child that makes them a medical patient for the rest of their life. Exclusion from women's sports comes with that. Some choices have consquences, you better use extra caution and consideration if you're making choices for other human beings.
One thing I will say though is I'm thankful I never had to confront this shit as a parent, I am empathetic toward parents out there who have to deal with this. But not empathetic enough to strip away opportunities for real girls who want to play sports.
That's exactly where I'm at with it. We gotta draw a line somewhere... draw it there and let's move on as a society.
I understand that there are one-off cases where it is reasonable that the kid who started gender reassignment shit at 8 years old be placed on the girls team even though he technically has a dick (that doesn't work). He didn't do it because, at 8, he wanted to rape teammates in the locker room or perv out or steal a scholarship. He did it either because he legitimately felt he was a girl, or, his parents pressured him into it/incentivized it and created this monster out of their own hubris.
I don't 'hate' that kid at all, and I can produce empathy for 'her'. If you want to meet in the middle and tell me I have to call her and her in order to keep my teaching license, fine, done. Agreed.
That said, empathy isn't the arbiter here. I also have empathy for the dumb as rocks idiot who can't play sports because he's too stupid to pass remedial math even though he tires his best in class. The fact that I feel kind of bad for him doesn't mean the rules should be abandoned and he should be allowed to play. You gotta show up to and pass your classes to play, it's a rule. It doesn't matter if we feel bad because you tried your best and still can't pass, shit happens and rules are rules.
It sucks that there are opportunistic assholes who will try to game the system to gain unfair advantages that ruin it for everyone else. Those people are always going to exist though, so we need to have rules to disincentivize them and/or negate the ability to game the system.
And as far as the, "not letting them play sports sends the message that they aren't the gender they identify as"... if you make gender reassignment a disqualifier just like satisfactory academic progress, it doesn't need to do that. It only does that if you make it that way. Just like a kid who can't pass classes isn't automatically classified as a retard... he's only a retard if he runs around telling everyone he is one.