One must differentiate between marriage and sexual union. Marriage is a legal concept, and is regulated by laws and customs of societies. For example, in some states it is illegal for first cousins...
depends. if there is children with in the house hold belonging to both parties of the case. if there is someone pregnant by the other party of the case. and if the two parties co- habitat the same...
It is okay to do it in most circumstances, but it is questionable if the document benefits the notary. Such as notarizing a quit claim deed to the benefit of the notary would not be a good thing.