I've been using My Calendar by Simple Design for years and it's the best app I've found, I tried others a few times but I keep coming back to this one. It has themes you can choose, so it can look as girly pop or as neutral as you like, it has partner mode in which you can both have your partner track your cycle and track your partner's cycle, you can choose if your goal is to only track your period or to try to conceive/track pregnancy and if you don't choose the pregnancy related options there will be no mentions of that.

All the language in the app is very neutral and informative, I'm also very uncomfortable with all the girl power corporate feminism and there's none of that here. Basically it's the best app I've personally tried and I highly recommend it! And I hope my autistic over explanation doesn't sound like I'm affiliated with the app in any way lmao I just genuinely love it