First off, we can discuss the fairness of transgender participation in competitive sports while not stepping on the people involved or purposefully going out of our way to offend / hurt them. I'm speaking specifically about our discussion here on TF.com. Regardless as to how you feel about trans people, they're people and deserve our respect.
Based on the numbers, we likely have one (or more) TF.com members who are dealing with these issues.
If you believe it's a mental illness, they deserve even more gentleness, do they not? I'm not suggesting you have to champion their desired outcomes, but we can, at the very least, be respectful amongst ourselves.
That said, I think transgender athletes should either have their own division or, until that's possible, compete based on their biological gender. As we learn more about this we can reevaluate as needed, but for now that seems the most fair for all involved.