The menstrual blood is a gift from the Goddess. Wes Penre, Second Level of Learning

This way, it's not only the second chakra that's involved in the sex act, but all the upper chakras as well, and it even opens up the eight to twelfth chakras outside of the body, which are normally quite closed while being stuck here in 3D.

Barbara Marciniak also refers to it extensively in her second book 'EARTH [Pleiadian Keys to the Living Library]'.
"Explore who you are, set an intention of what you want, and make sure the time you spend expressing your sexuality is a time of joy. Sex is not about performance; it is about the most intimate connection and sharing possible..."