It's pretty simple to me. I'm a curious science-minded sort of person. It's a natural function of the body that while described in a general sense is never gone into in detail and almost no one talks about. I don't have any so how am I going to find out otherwise? The amount of knowledge I've accumulated from my girlfriend in this respect is enormous. Sure it probably doesn't make much of a difference in my daily life, but it makes me significantly less ignorant than I used to be.
Likewise a woman's menstrual cycle has a massive effect on the hormone levels in their bodies and can have some telling effects on people in a variety of ways, sexual and not. If nothing else it certainly helps to have a good idea of your SO's cycle so you can try to make them feel better until it passes. "Why'd you get me flowers?"
No need to be obvious though:
"Because you're having your period."
as it can tend to kill the romance.