In my case, a good candidate for that function would be the app Nice (Sex Tracker by Nice). It’s an app that, well, let’s you keep a register of your sexual life, alone or in company. It also allows you to specify what kind of intercourse you had, with which partner, and put it in a calendar view to have a better panoramic of your sexual life. It also allows you to track partners, possible STI risks on each one, and other details. Then, it elaborates statistics about your sexual activity.
Although it allows you to lock it using your biometrics (something that we’ll be able to do with all apps on iOS 18), I think it’s a good candidate to be hidden on your iOS device. But no one has access to mine anyways.