I'm thinking of monitoring my girlfriend's menstrual cycle
Why would you track it?
It's sort of like an indicator of her attractiveness.
It is rather strange to track your partners cycle for that reason. It might make more sense to find out when she might get pregnant than to track how attractive she is.
It doesn't matter, if she wants to get pregnant then I want to know when she's most fertile.
I'm glad that you respect your girlfriend's wishes regarding pregnancy, but this is a personal matter for her, and you probably shouldn't be tracking it so closely. It is best to just treat her with kindness regardless of how she looks or feels on any given day.