I'm not sure what the historic reason for not yogaing while menstruating is, but the current explanation is that it stops the natural downward flow of the blood (and tissue), and also the redirects the natural downward energy flow that comes with menstruating. I'm not currently a big believer in the energy theories like tai chi, so I don't follow that advice. I also don't worry about changing the direction of the flow on my period since its only a few tablespoons (which is what those pamphlets you get in sixth grade say). Wouldn't sleeping interfere with the downward flow?
But then there is a difference in energy levels (the whole being) at different times of the month, so that is another reason a woman might not want to do vigorous poses then. Bryan Kest actually says every woman should do what's right for her. But, that said, doing restorative postures for 5 or 6 days a month is surely not a bad idea.
Breathing into the groins. Rodney Yee says this, especially in Yoga for Athletes. This is often laughed at because some of us are very literal and we only have one groin. But what he's talking about is taking deep breaths and thinking about the oxygen and air going into the groin area (or whatever area they are trying to relax). It really isn't bad imagery and helps you focus on the area you are trying to stretch and relax. It brought me out of the moment so to speak, because it sounded funny the first time I heard it.
As with anything, your milage may vary on your opinions in these matters.