A lot of dudes keep track of their girlfriend's periods, because every month there is one "safe" weekend where you can have unprotected sex and get away with it.
This is sometimes called the "Rhythm Method" or "Fertility Awareness" and a lot of men have self-interested reasons for wanting to know this about a woman.
There is a history of apps optimized for this purpose. Some wholesome for married couples to use, and some for skeezy bachelors who want to track multiple women.
It's not crazy. Sex is one of the most fundamental and powerful desires we have. What's actually crazy, in a mostly positive way, is how much control we've managed to achieve over it at individual and social level.
This is sometimes called the "Rhythm Method" or "Fertility Awareness" and a lot of men have self-interested reasons for wanting to know this about a woman.
There is a history of apps optimized for this purpose. Some wholesome for married couples to use, and some for skeezy bachelors who want to track multiple women.
https://gizmodo.com/iamaman-period-tracking-iphone-app-for-s...
https://jezebel.com/another-innovative-menstrual-tracker-app...