They can, obviously. In the mixed sex pond. They can all go in there together and have a lovely time.
The legal issue is that you cannot call it a women only space - which uses the legal sex exemption - and let some men in. Because you've then fucked the exemption you were using to exclude any men at all.
Which would mean you'd have to call the women's pond a mixed sex pond and the mixed sex pond a mixed sex pond, and the men a single sex space. And then deal with the discrimination claims from women.
And morally, obviously, you cannot stand nobly waffling about inclusion and kindness and wellbeing and socialist utopia when you have fucking excluded women who need single sex spaces to have any access at all and are just pretending not to see what you've done.
And obviously, to anyone with a brain, excluding women from anything so that men can have everything is a sexist dick thing to do and support.
Make it three mixed sex ponds - then whatever. I'll wait for the potential discrimination claims and still think this is misogynist exclusionary dickery, but crack on. You do you. It's the lying that needs to stop.