A few cases in which transgender people, male to female, have had noted impact on their sports have prompted this.
Renee Richards was one of the first of these. A mediocre (at best) male tennis player, he transitioned to female around age 30 and she won decent money in women's pro tennis tournaments.
It is also intertwined with the issue of intersex athletes like Caster Semenya who compete as women. They have naturally high levels of testosterone that would otherwise be considered using performance enhancing drugs. So these people make the definitions of male and female athletes very difficult. How do sports define "female" for purposes of equitable competition?
Was it "fair" for 6-1 Renee Richards to compete as a woman after sex reassignment surgery and hormone therapy? She reached #20 in the world as a woman. Did she deprive someone who was born biologically female get deprived of a top 20 ranking and tens or even hundreds of thousands of dollars in earnings?