Genesis 2:24

‘Therefore will a man leave his father and mother and cleave to his wife, and they will become one flesh.'

It is because of this close relationship between a man and his mate that that relationship supersedes that of his parents. When they enter into sexual union they become one, bound in a relationship closer than any other. Family loyalties still hold, but the loyalty between a man and his wife is primary. Notice that sexual relations are treated as normal and good (in spite of the euphemism ‘cleaves'). There is no suggestion anywhere in this account that sex is to be seen as somehow sinful.

The fact that the man is said to leave his father and mother indicates that here a new unit is forming. There will, of course, still be family ties and responsibilities, but essentially by marriage the man is stepping out to form a new unit with his wife which is unbreakable, and complete in itself. The impression given is that a man will have one wife.