I never have sex while I have my period. Even though it is natural, I just think it's icky, I hate the mess, and I hate the sight of blood in general. Moodwise, I'm grumpy and withdrawn, and the first few days I have terrible mood swings. Things like people looking too happy drive me into a rage and those animal shelter commercials will send me into tears. I can't imagine wanting the intense human contact that comes with sex.
Master hates the sight of blood too, so sex is a no go for him either. He generally takes it as a week of blowjobs for him.