In most cases it’s not appropriate for a random person to bring up, but there are definite contexts in which talking about someone’s biological sex (whether referring to a trans or cis person) is medically necessary. Females and males don’t have the same medical risk factors for everything, and implying that gender identity changes that is just incorrect.

And while I haven’t met a trans man who “cheerfully refers to himself as biologically female” (because I don’t have many trans men in my personal circle of friends), I am friends with a trans woman who refers to herself in a matter of fact way as “biologically male”.