If you're fit to work, then you work. If you're too unwell you're not. That's the threshold, and this is the case whatever the nature of the complaint.
If you have a diagnosed condition then, if this becomes a problem for your employer, it's possible you might be protected under sex discrimination criterion of the EA 2010. However, anyone in this position should take legal advice ASAP because I'm only flagging a possibility. It's not a given, and as ever this will depend on individual circumstances. If employers notice specific patterns of absence and there are not reasonable adjustments on record, then it's very likely you'll be asked to justify it. And that's fair.
This is the overall issue I'd have first and foremost with any suggestion of leave for this issue. Each case is judged on its merits: a blanket 'menstruation leave' will do nothing but put women even further back in terms of our workplace rights, progression, and the usual glass ceiling.
There are many fights still to be won, medical misogyny not the least of them. Were this not the problem that it clearly is, then the whole thing might just be less of a problem in the workplace. Some women have to fight tooth and nail to get their hands on much-needed HRT, and an endometriosis diagnoses took years for me - my doctor refused to believe it was anything other than a hernia.
This is a complex issue and takes more unpicking that a suggested 'solution' that would cause more problems than it prevents: not that this is being mooted seriously anywhere other than on Mumsnet.