Well, wanting to predict ovulations probably means the user is trying to conceive. An unscrupulous employer might want to know that, and try to replace that employee.
Also, some women have health issues involving their reproductive tract, like PCOS, fibroids or even just PMS. Data can help them better understand and manage their symptoms.
https://www.cdc.gov/reproductivehealth/unintendedpregnancy/p...
You'd just start seeing your performance evaluations come back with "needs improvement". Little mistakes or violations of policies that were previously ignored result in write-ups. Six months later, you're laid off with a couple other folks, with a hole in your income that makes it hard to hire a lawyer on a long-shot legal suit.
People get fired for being pregnant daily. As long as their boss doesn't do something abjectly idiotic like sending an email saying "you're fired because you're trying to get pregnant", there's very little recourse.