OP,
You are wrong to say this is a new low or even a low at all. Allowing transgender women into competitive sports is a topic that is appropriate for runners, regardless of what the current sport the story is about. So, good show Letsrun.
I am 100% behind what any transgender person wants to do (within the realm of legality of course). I'm ok with them being legally listed as which ever sex (yes, sex) they want to be listed as. I am ok with them defining their gender and sexuality however they wish. I don't care what they wear or what they want me to call them. I will gladly call anyone whatever it is they want to be called.
What I won't do is support a person who was born a male who then wants to compete as a woman in a women's sport. Unfortunately there are a lot of bigots with that position who just want to deny transgender people anything, but that's my only restriction. It doesn't matter if a 22 year old takes pills to lower his testosterone. He's done growing. The advantage is already there. Completely unfair advantage.
I suppose that technically I MIGHT be ok with a transgender man competing in men's sports, but since they take testosterone to help them achieve the look they want, I suppose that would have to be closely monitored. Probably just easiest to say that the sex you were born with is the category of sports you can compete in.