It's a pretty nuanced topic and the conversation is about where we draw the line in terms of genetic advantage. It's true that we decided long ago to have more lines for MMA and boxing than we do for say basketball, but that doesn't mean we should or shouldn't draw a line in this case, neither example can really be used as justification really because these things tend to be treated individually from sport to sport. The current problem is that this is mostly unexplored territory for sports and society is still trying to decide where that line should be drawn in this instance. A big difference is also the fact that tall people in basketball are genetic anomalies but they were born that way. The advantage trans women have is man made. Some would argue it's therefore closer to the advantage gained from steroid abuse than it is a genetic birth advantage. This is of course also a big exaggeration, there's no evidence so far that any trans athletes are transitioning purely for the competitive advantage and it's ridiculous to suggest so at this point. But the advantage is still there, and it's significant, and it's man made, and it risks muscling out non-trans women from high level sports altogether.

There doesn't seem to be an easy solution. In women's leagues they have a huge advantage, in male leagues they have a huge disadvantage, and there likely isn't enough elite level trans athletes for a 'trans league'. We've yet to find an elegant solution either way.

For the record, I support trans rights and think they should be treated as whichever gender they choose to be in general by society. Who someone is is none of anyones business and I respect that. This issue does infringe on others though (born female athletes) so deserves a bit more discussion to come to a fair solution for all parties.