It's not a question of same-sex affecting their individual marriages.
For thousands of years human society has affirmed that marriage between one man & one woman is the natural order for procreation and the best environment for the raising of children.
I think you have that right. The larger issue might be stated as that we want government to stop social engineering. They don't know enough to overturn eons of practical development. This is just the tip of the iceberg. We want them to stop bossing us around by thinking they should tell us how many children to have, what to eat, what to drive, which light bulb to buy, and make us switch to toilet that don't flush and washing machines that break our backs.