Please explain how "male" is the default here?
Menstruation is a biological function of the default healthy female. On average that means a healthy female is affected in one out of every four lessons.
If females were the default, scheduling principles would have developed taking this into account from the outset, ie when regular swimming lessons were first offered to the public.
Please note, the default is not just the average male, but the average healthy male.
Flexible schedules would also benefit those who regularly miss lessons because of doctor's appointments or not being well enough to attend because of their disability.
This is especially relevant to swimming because it is an activity that is particularly beneficial to people with disabilities or chronic illnesses and that many more of them can engage in than running or cycling for instance (like the OP's daughters).
*Surely it is down to affordability and viability of a finite resource?
If they offer flexibility they will have to run with spare capacity. That will cost money.*
Yes. Reserving seats in buses for the disabled or only buying accessible train carriages also costs more money. So do programs designed to address inequalities faced by other groups. Pretty much everything costs more money that takes anything other than the default user into account.
And why are you only considering swimming? What about ballet, gymnastics, tennis...? Are you proposing subsidising/ increasing prices for all sports?
Because the OP has a specific issue with swimming (which her daughters engage in for health reasons). That's because full body immersion in water is one of the few leisure sports activities where menstruation is a hygiene issue that limits management options, typically to using tampons.
The OP has not made a complaint to the provider or demanded anything. She has pondered a particular problem she is experiencing, has identified it as a potential inequality that affects only females as a class and has come here to ask for possible solutions that respect her daughters' boundaries around their own bodies and that may redress the problem.
She has not demanded that others pay more, she has considered whether that may be a viable option. There is a difference.
As for the notion that feminism must take all other groups into account when seeking to redress an inequality that affects females as a class - no, it really doesn't.
Feminism is not egalitarianism, it is the fight to free females from oppression by males. Until this is achieved, feminism also seeks to address and redress inequalities suffered by female people as a consequence of living in a patriarchy.
Intersectionality means taking other axes of oppression into account than may affect females, such as age, class, disability, race or religion.
That does not mean one pushes through measures to the detriment of others, but girls - not just boys - also belong to the bottom third of society, ie are affected by poverty. So if we could find a solution that works for girls from poorer backgrounds, it would also work for boys.
Although it can be hard to put the needs and interests of females first, this is what feminism strives to do precisely because our needs and interests have historically not just come last, they have not featured at all.
If the plight of boys from poorer backgrounds is paramount to you, or even just fractionally more important than an inequality suffered by girls (whose parents can after all afford to pay for swimming lessons), then there is also a strong anti-austerity campaign that you can join instead of or in addition to a feminist campaign.
One does not have to limit oneself to one cause, but it is entirely legitimate and justified to do so. So if women here wish to put women and girls first, that is their prerogative. If you don't wish to do this, that is yours, but neither is an inherently wrong or morally questionable choice. These are both valid choices we can make.
However, coming back to the swimming lessons, I would add that a flexible schedule that would allow customers to make up for downtimes, would benefit not just girls, but also swimmers with disabilities and those from poorer backgrounds for whom block booking is more economical than paying for single lessons but who would nonetheless find it hard to justify paying for lessons they cannot use.
Such systems are in place in a range of different paid for activities. These systems have rules. Such a rule could be that one could reschedule one lesson per block for instance or that one would be charged if the lesson is cancelled within less than 24hrs for instance, but could be rescheduled if 48 hrs notice are given.
There are a number of options one could consider that would not mean raising prices for all - for instance a tiered system: book a fixed-schedule block for the lowest price, book a block with limited flexibility for a medium price, a block of maximum flexibility for a higher price and individual lessons for the highest price.
As the issue of an inflexible schedule also affects people with disabilities, the numbers of customers who could benefit from a more flexible approach would therefore be much larger than just menstruating girls who for cultural or physiological reasons cannot or for personal reasons do not wish to use tampons.
If I was in control of a swimming pool, I would want to trial a tiered system across all my classes, including swimming lessons. I'd market it to people with disabilities and to the time-poor customers of today who wish to get fit but cannot always commit to a fixed, weekly schedule (such as my DH who travels a lot for work) as it would allow customers to choose whichever booking schedule suited them best. The higher pricing would compensate for the problems identified by PP that flexible scheduling could create.
As far as I can tell, this doesn't seem to have been tried, so proclamations that it would lead to ruin or an end to public swimming lessons are premature in my view.