The Ortthodox Church bases its ideas of marriage on Ephesians 5:22-33. I quote the verse 32:

This is a great mystery, but I speak concerning Christ and the Church.

Hence, marriage is seen as a living icon about the relationship between Christ and the Church. This is also part of the basis for the Orthodox position against same sex marrriages, since there is only one Christ and one Church, not two Christs or two Churches