Marriage - a blend of instinct, economics and religion?

The only act in this whole realm which can be called instinctive in the strict psychological sense is the act of sucking in infancy.

I do not know how it may be with savages, but civilised people have to learn to perform the sexual act. It is not uncommon for doctors to be asked by married couples of some years' standing for advice as to how to get children, and to find on examination that the couples have not known how to perform intercourse.