Why are people freaking out over trans people not allowed in womens or mens sports, Vice versa

I mean like, why are people acting like it’s the end of the world that a trans person ain’t competing in womens sports or a woman isn’t competing in mens sports. What is so absolutely horrid and awful about this?

Edit: thank you for the responses, I just wanted to get the mindset behind why it even is a thing, I know Reddit is full of liberals, still valuable knowledge. Thanks for the replies, regardless if I agree or disagree with you.