Mostly it's that you haven't pointed to a specific portion of the ruling or the law. Until this happens, I'm going to assume this is mostly wishcasting a preferred outcome on to the situation.
Feminists generally don't mind comparing the sexes to each other in terms of relative advantages, so it's weird that you've scrupulously avoided doing so here. In the scenario where "all men are excluded" from the women's pond, they are at a disadvantage relative to the users who have access to that pond.
Are you messing with us?
If there are facilities where being separated on the basis of sex is reasonable and appropriate, then they can be separated on the basis of sex - but in doing so, they must adhere to sex as a biological reality. They cannot make exceptions for some people of the opposite sex.
In this situation, there are two ponds: One is a pond for males and the other is a pond for females. They've historically been separated by sex. Over the past several years, the management has made exceptions, and have allowed some males to use the female pond, provided those males said they had a special gender essence in their minds.
The recent SC ruling made it clear that they cannot make exceptions of that sort.
When it comes to toilets, changing rooms, showers, etc. it's pretty clear. If it's a single use facility that doesn't have any shared spaces, it can be designated unisex, no problem. So if a small restaurant only has two completely enclosed restrooms, they can both be designated unisex. But they can't make one of them for males and the other one "gender neutral" or anything like that - if one is set aside for one sex, there must be a facility for the other sex, and they have to enforce that sex designation.
In this particular case I end up on the fence. They're swimming ponds that have historically been separated by sex, so there's tradition there. On the other hand, most ponds, pools, lakes, etc. aren't separated by sex. As I understand it, neither of these ponds allows nudity. So I think there's a plausible argument to be made that both ponds should be available to both sexes.