Because people's perception of someone's physical sex is based on a usually quite cursory glance. So any attempt to segregate spaces based on born sex will lead to those who don't look like their sex being excluded or harassed.
Any attempt? What about prisons and hospital wards? Where people go is based on much more than a 'cursory glance'. Do you agree with these spaces being sex segregated?
And increased hostility towards trans people, which is already happening, will force trans people back into the closet and make people think twice about being gender nonconforming.
Gender nonconforming people have been around forever. And what is causing hostility towards trans people is the entitled actions of some of them, such as male people wanting access to female only spaces. If they were content to accept that they can't change sex, most of the problems would disappear.
You can see that already happening with the moral panic about whether drag queens should be allowed around children or not.
Drag queens aren't gender nonconforming people. They are male entertainers and performers who make a living out of portraying themselves as a grotesque caricature of a woman. This misogynistic portrayal of women is unsuitable as children's entertainment. I'm sure you understand why.
An association is now being made between being gender nonconforming and sexual predation.
Well, if you equate 'gender nonconformity' with a sexualised and misogynistic portrayal of a woman by a man, you are making this association yourself.
If that continues then it will not be safe to be gender nonconforming, just as it wasn't safe to be gay when the same trick was pulled in the 80's and people got queerbashed on their way home from a night out.
Male violence. It's a problem. But men need to sort that one out. Women aren't causing it, so how do you expect us to stop it?
It will also of course lead to trans people, and especially trans kids, doing everything they can to get medical intervention as young as possible in the knwoledge that they leave it too late they will never be accepted as their aquired sex.
Well, maybe it's time for people to face the reality that they can't change sex. Stop telling children that they can. And it's testosterone that causes the irreversible changes, so there's no excuse for giving girls medication at a young age. And for boys who don't go through puberty, genital surgery is more complex. So there are downsides to early medication for them as well. And that's even without mentioning the 80% desistance post puberty if children aren't medicated.