I don’t know enough about the topic, so this comment will be a blind leading blind. Please don’t hesitate to correct me.

My understanding is that chasers would likely not be okay with bottom surgery and they would be acting more as if that person was their assigned at birth gender, but it’s also not the point. The point is that they are fetishizing trans people. It is understandable that some people have a preference, however in the case of chasers it is a division between cis and trans. In other words, they don’t see trans people as just people, but rather as trans in the sense that it is something separate from normalcy. Or more simply: they see trans men and women as separate from men and women.

Why don’t they look for non-binary people? Some probably do, and I wouldn’t hesitate to also call these kinds of people chasers. In my mind that is still fetishizing a specific group of people, though the reasoning is probably different.

Context: https://www.scribd.com/document/899239966/Henry-the-Green-Engine-1951-Scanned-by-ThatERTLGuy#page=64 page 60 of the book, 64 of the document.

Written version in case someone doesn’t want to click on random links:

The Guard’s flag waved, his whistle blew, and they were off. Soon in the distance they saw the bridge. There were the boys and they all had stones. “Are you ready, Henry?” said this Driver. “Sneeze hard when I tell you.” “Now!” he said, and turned the handle. “Atisha Atisha Atishooooooh!” Smoke and steam and ashes spouted from his funnel. They went all over the bridge, and all over the boys who ran away as black as ####.