To posters who think it's something medical, etc, I'm going to go Occam Razor. A PP somewhere theorised that she keeps her periods hidden from him. A diaphragm would do that. Or, even simpler: he's just too stupid to realise why she won't see him a few days a month.
Well, yes. That's rather boring, but the most likely, I admit.
So, perhaps someone should announce it on a men's forum:
If your girlfriend tells you she doesn't menstruate, she's lying! She just does it while you are away!
(There was this thread where people argued whether there are men who don't watch porn or whether all men who claim this are liars ... let's just say, men are physically completely capable of not watching porn. Women not menstruating is a lot less likely)