There are those who say to trans women, “You have/had a penis, that makes you a man, end of story.”
I think we can see that there’s an agenda here asides from a love of truth, because even if this was true, it’s just downright disrespectful to say it, especially when you know it’s going to hurt. Besides, trans people are not deluded: we all know we have or had a certain body, we simply don’t believe that defines the rest of us.
But supposing for a minute we were to entertain this proposal seriously. My answer would be:
How do you define a cat? What is the single aspect of a cat that makes it a cat?
How do you define an orange? What is the single aspect of an orange that makes it an orange?
How do you define a willow tree? What is the single aspect of a willow tree that makes it a willow tree?
These are all biologically-related examples, and I think that fits nicely because gender is also in large part biological. In biology, there are no absolute defining lines between one thing and another, as that’s a thing which humans invented. Of course, you can say that the ability to interbreed defines a species, but if so, what defines the difference between an Asian wildcat and a lynx?
When we define a cat, an orange, or a willow tree, we can’t take a single aspect of each and say that is the one thing that defines them. A cat is not any animal that has paws; an orange is not any fruit that is orange in colour; a willow tree is not any tree with long leaves. If you wanted to help someone understand what a cat was, you’d have to describe it, using many different features. The sum of these features would constitute your definition.
The same goes for gender. Sure, it may seem that gender is a bit more clear-cut, but it really isn’t. A person can have a birth defect and be born with no genitals, yet still be identifiably male or female in other ways. If someone had such a birth defect, you’d depend on other aspects of them to make a definition – which shows that you never did rely purely on genitals for your definition in the first place. And I think if you say otherwise at this point, you’re simply being disingenuous.