What is a Marriage?

I don't think I can put it any better than author Dean Sluyter said it in his
book, "the zen commandments".
"Firmly committing to one partner under all circumstances
(in sickness and in health, come rain or come shine) is like setting a camera
on a tripod. It provides the steadiness necessary for a long exposure, which, even in near darkness, eventually produces a clear view, awash with light. See what develops: the camera of awareness records the true face of the partner, which is also our own true face and the face of existence itself. 
And in this way, love not only stays alive over the many years of a marriage, but flourishes. As awareness gets clearer, love gets deeper -- turns out, in fact, to be bottomless.
Such a process deserves, if anything does, the label "sacred".
This is what those who condemn marriage between two people of the "wrong" gender or color or religion fail to grasp.
Whatever reveals the infinite is sacred, and any union based on devotion does just that.

Throughout humankind's history people have been persecuted for having ideas that stepped outside the norm, such as the earth not being the center of the universe...
Then the ideas eventually get accepted. Why not save ourselves the despair and approach things with a more open-minded approach.