With all due respect,what poor analogies you've given there. You're implying that women are weaker than men,even disadvantaged. Some feminist would call you 'sexist' for that.
I tried to help you out, I gave you some info, which you even had the decency to absorb, you were doing quite well there for a moment.
A point even, to you, for thinking about it, and imagining yourself in someone else's place, for a moment.
But darn sorry, no, no feminist would say that women only swimming sessions are 'sexist'..
Or certainly not in the world as it is today.
They would say that women only sessions recognise that men have historically, and up to present day used their averagely greater physical strength, and tendency towards aggression, or threat of such usage to make women feel uncomfortable.
Women tend to be (but are not always) less physically, powerful than men.
This muscular strength is a physiological biological advantage that men have historically used to 'be in charge' to wield the most power.
A hang-over from the days of 'might is right'
Once civilisation struck (in some parts) it was recognised that men had hitherto abused this greater physical power, and aggression often manifesting as violence, physical, verbal, or even sexual violence, to gain all sorts of other powers,
Financial, social, political, reproductive, and all the rest of it.
Feminism seeks to have recognised, and redressed this unequal power balance, between the sexes.
And tries to make things more equal.
As you recognised yourself, some women (with good reason) sometimes feel threatened in the presence of unknown men, either as a result of their own lived experience, or from the historical experience of other women who have had various violences done to them.
Ergo they are naturally (or you could argue have learnt to be) more wary, and therefore feel happier and more relaxed in persuing their sporting activities. ( Particularly when clad only in minimal clothing) Away from the male gaze, judgement, or potential unwanted attentions.
Many of these men of course wish these women no ill, at all.. But how are they to tell the difference?
Enough men have, over history, and even still now have wished, or done them. ill, enough times for them to prefer single sex activities.
It's unfortunate that this 'guilty until proven innocent' attitude has arisen.. Within large parts of the female population. But it is as a direct result of (some) mens behaviour.. It's not as a result of women behaving badly.. They are just being themselves..
I'm not sure I've really got the energy to explain any further.
If you're really not getting it yet, I'm not sure there's any hope..
Perhaps imagine you raised your head from your powerful front crawl, up and down the lanes, to discover that the rest of the pool is inhabited only by women..
Perhaps they're tidy engaged in a breaststroke, (the technique of which meets your approval naturally)
If you happen to notice this gender imbalance at all, would it in anyway perturb you..?
Make you feel uncomfortable?
Probably not, you might even consider yourself lucky to find yourself in such pleasant company.
A female however, finding herself surrounded by a pool full of guys, that she didn't know, might feel otherwise, yes??
So no, it's not sexist to have women only swimming sessions, it's sexist not to recognise why some women may feel more comfortable attending them.