It is an accurate predictor of sex well over 99.9% of the time. And even in the cases when it doesn't, it has nothing to do with trans people. You repeatedly duck this simple point.

But ah!! Finally we get to the real reason why you're so indoctrinated on this issue!! Your kid is trans and you love your kid. And you think spouting this nonsense is being supportive. That's understandable. Sincerely. But consider that it's not necessary (or helpful) to deny reality to support your child.

If your kid is "gender non-conforming", support them, as I'm sure you already do. Let them express themselves in any way they want to. As I'm sure you do. Let them know that their expression is normal and healthy. As I'm sure you do. But then let them figure it out for themselves. Know that in 80-95% of cases (https://onlinelibrary.wiley.com/doi/full/10.1111/j.1743-6109.2008.00870.x), your child will grow out of their dysphoric feelings all on their own. And if your child is one of the cases where they don't, then that, obviously, is fine too. Help them with their transition, support your child, as once again, I'm sure you already do.

But while your child's needs are important, and their validation in society is important, so are the needs and safety of women. So the argument that I'm making, and have made from the very start, is that we need, as a society, to figure out ways to resolve this overlap of needs without simply telling women they can "easily ignore penises unless they're hard and being waved in their faces." Do you really not see how loathsome you sound when you say something like this?

God, it's such a relief to understand the real reason why you're like this. I'm actually really glad I didn't give into the temptation to block you. I hated imagining there was somebody out there so indifferent to women's rights simply because he didn't care about women's boundaries.

You want your kid to have a good life. I do too. You may not believe that, but it's 100% true. But I want (I'm presuming her?) to live in a world where their happiness doesn't rely on forcing everybody on the planet to deny reality. That isn't going to happen. And it's a fragile, insecure life where they think everybody who tells the truth hates them. I've seen it in some of the trans people I come across.