Let's Get Metaphysical: The Etiquette of Entry

From both our personal experiences of our own varied sex lives, and in our work in sexuality with many other people, it seems pretty clear that really letting someone into an internal space in your body, or going into someone else's insides -- which we know might sound a little gross, but that is what's going on with this stuff -- is a fairly big deal for many people. So, what might make sexual entry different from other sexual activities?