Marriage and Mating!

I used to hear that marriage is a part of human nature, and other animals out there are mating. What that has to with religion? I thought marriage was design to facilitate a family, despite this partly true historical context of marriage today people can adopt children or obtain children from egg/sperm/birth donor technology have families without marriage yet the have several civil union (mating I think!) anywhere they meet.

I am not being facetious, I really want to know; Is the difference between ''marriage'' and civil union (mate) really just the name?

What are their implications on homosexual and polygamism? Perhaps legal marriage could be replaced in law with civil unions(mate) regardless of human or other animals and marriage could be something of relating to the traditional culture passed down in the church.