Same sex. I think someone in favor of transgender people in sports can start begging the question and ask about all the other factors that make sports unfair for people of different morphology. If you take M. Phleps as an example. He trained hard but he is also built like a freakish combination of a frog and a dolphin giving him a genetic leg up on men and the world.
We could tilt the conversation into languages, math, science, theater, etc. There are people in this world that just do things better than you or I because their genetic coding allows them to do it better no matter how much we work on that same skill. I cannot learn five languages fluently with nearly the proficiency of a couple of people I know. It then becomes up to our society to put value on those talents. Swimming fast in a pool or hitting a tennis ball over a net doesn't do much for society as a whole but a lot of people enjoy the competition.
I know I wouldn't be so Spock like about it if it were my daughter competing against the trans swimmer. There should very much be an asterisk on these records being broken if we are talking strictly "women's" swimming titles.