That is the way I look at it, Tater. I fail to see how same-sex marriage could make a committed marriage between a man and woman any less honorable or important.
Some ideas of why folks get up in arms about same-sex marriage:
1. Their own marriages are unhappy and they don't want two guys or two ladies to have something they don't.
2. They are afraid their kids or grandkids will somehow catch a "gay" virus and end up not giving them and grandchildren or great-grandchildren.
3. They believe in a literal understanding of the Bible, so they are happy to take the verses that discuss same sex activities and act as though those verses are talking about consensual behavior between two adults in a committed relationship, but at the same time, won't get too upset about Aunt Sadie's six marriages that ended in divorce, because "times are different now".
4. And perhaps some of them are afraid that gays and lesbians might actually end up being more successful statistically in keeping marriages together than heterosexuals have been for the last several decades.
Probably plenty more, but those come to mind...