of course it is - anything public will welcome many protected characteristics - men / women / disabled / different races / different faiths / etc.
you can also combine two characteristics if relevant - e.g. disabled women.
however you can't legally label something as only for one and then allow in those which stops it being for the first.
So, you can't have something for those who are disabled only and let in those who are not disabled - similarly you can not have something for women only and let in men.
If you want women and men then it must be labelled as for both and you can't exclude any who are women or men - i.e. straight men as well as 'trans women' would be allowed... and it would no longer be for just women...
of course, all of this is perfectly well known by those in charge at the ponds, and of course those posting on here, where it has been analysed to death many times... however, there are those who still think that somehow they can confuse the discussion by pretending that this is somehow a gotcha which allows men into women's spaces...
if a space is women only - letting in 'trans women' means letting in men
- it is no longer a women's only space - it is now mixed sex
- you can't then stop all men from joining in - because you are allowing it to be mixed sex - so even non-trans men will be allowed in.
the ponds have a pond for men / a pond for mixed sex (which includes men) and a pond for women (which does not include men) - any men in that pond are there illegally.