A Sexual Power of Attorney is a document we make before we need it. A relatively new document, it outlines for people (usually the elderly) who are entering into institutions, what our choices and attitudes are towards sex.
The reason for this document is that for the first time, nursing homes and hospitals are gaining patients who grew up with a different set of sexual beliefs. As we age, and need further care, we need to define for ourselves what is important to us. If you are facing dementia or other similar circumstances, if you do not have a sexual power of attorney, than if you find yourself in a sexual situation; it will be assumed that this is something you didn't want, and you will be removed (sometimes forcibly) from the situation.