"Echad" in scripture is not solely connected to marriage. For example, it is used here to describe the unity among those who were building the tower of Babel. Not only unity in purpose, but also unity in language at that time.
Gen 11:6
And the LORD said, Behold, the people
is one (echad), and they have all one (echad) language; and this they begin to do: and now nothing will be restrained from them, which they have imagined to do.