Because most sane and "normal" human beings wouldn't "go there", thinking that polygamy, pedophilia or bestiality is synonymous with same sex marriages. It stops at consensual human beings, not related and of legal age. The world is changing, and you can't stop it...your only option is to accept and adapt. Sorry.
Granted, but words have meanings and if we redefine the word for one aberrant group, the flood gates are open for Mormons who want to practice polygamy, NAMBLA, Muslims that want to marry off their daughters at age 8 and lonely shepherds.
Far fetched? Yup, today, but 40 years ago, gays who lived together knew that they wouldn't get a Priest of a Justice of the Peace to perform a marriage ceremony for them because their relationship couldn't be considered a "marriage".
Sorry guy I may have to adapt, but I don't have to accept.