Arguments about bone density are some of the more spurious. There is a large amount of variance in bone density among women. For example black women have on average similar bone densities to the average white male. And yet they still aren't competing on the level of the average white male. And neither are trans women. 

But if you truly think bone density makes a difference, I fully expect to hear about you campaigning for sports divisions based on people's DEXA scan results. Otherwise it has nothing to do with fairness, safety, or any other argument -- it's just your presumption and prejudice about trans people speaking. 

I don't think you will though.