According to the most ancient historical documents marriage is as old as humankind and began as a union between a female and a male. This is a unique union and serves a purpose that none other can accomplish, procreation. For literately thousands of years marriage has been defined as a union between a man and a woman. Marriage of a man and a woman provides, potentially, the best possible environment for nurturing children.
In recent years a movement has arisen promoting the expansion of marriage to include homosexual couples. That concept radically changes the meaning of marriage. Originally the primary prerequisite for marriage was having two people of opposite sexes joined for the purpose of procreation. Now we are being told the only prerequisite should be “love”, which doesn’t mean anything because love cannot be measured, proven or unproven. Therefore, a group of several males living in a single residence could claim to love each other, arrange a marriage ceremony, be wed to one another, and become a family. Using this method, most people could qualify for marriage benefits, including tax breaks. It is also conceivable that legal marriages could include a person with a pet or barnyard animal (such marriages have occurred). Since our modern society is constantly pushing to eliminate morals (for example this often repeated sentence: “You can’t legislate morality.”) on what grounds could polygamy and human/animal marriages be prohibited?
I see no rational reason to change the meaning of marriage but changing that meaning will undoubtedly open a can of unpleasant worms.