This really isn't an issue if the woman is using an alternative menstrual method, like a menstrual cup. The menstrual cup creates a suction when inserted into the vagina that keeps everything completely sealed from the outside world. I use one and I don't even think the smell of menses passes through while it's in. Before I started using a menstrual cup I wasn't even willing to masturbate during my period... now I'm able to masturbate normally with no mess and no risk of leaks. I should think oral would be just as carefree.