Additionally, cisgender people like me sometimes affirm our gender medically too. Does this make us in some sense non-biological?

For example, as a cisgender man of 61 years, I'm bombarded with lots of targeted advertisements. Besides offers to join AARP, use hair-growing medication, and take pills to enhance erections, I often get ads touting the benefits of taking testosterone supplements.

I'm not clear on all the medical science, but my general perception is that as men age, our serum T levels tend to dip. This can mean that building or maintaining muscle mass becomes more difficult and that the sex drive lowers somewhat.

Testosterone is therefore sometimes hyped as a solution. "Take a supplement to be strong and virile." I know of an online service that offers T supplements to older men after a "consultation" with an MD that consists of nothing more than filling out a questionaire. The company even says no blood tests are required.

I don't take T supplements, and given my age, my serum testosterone levels are probably significantly lower than they used to be. If they aren't already lower than "normal," they will be eventually.

So, if I were to take T supplements, would I no longer be a "biological" male? I can hear objections from the anti-trans crowd already. "No, low serum T in older men is an ordinary human phenomenon. Your taking a supplement is just a private medical decision that has no bearing on your gender."

Really? OK, then. Experiencing oneself as transgender is an ordinary human phenomenon too. As far back as we can peer in time, and across multiple cultures, we see trans humans existing in one way or the other.

So, why is a trans man taking T less "biological" than an older cis man like me taking T?

Testosterone supplements are fully biological, of course. Both categories of man are fully biological, of course.

But then, as you illustrate well, people opposed to trans existence use the label "biological" to actually suggest that trans people stop existing. That's what they actually want, and they aren't coy about it.