The argument that opening our minds to same sex marriage might lead to opening our mind to other options has some merit.
That's pretty much my point in this thread.
Imagine a time-honored apple pie recipe. No one disagrees that if you follow the recipe, you get an apple pie.
Now someone comes along and tweaks the recipe. They use pumpkin instead of apples. However, they insist that it still be called an apple pie.
The apple pie traditionalists object, complaining (among other things) that if you make apple pie with pumpkins instead of apples, it's not apple pie anymore; and that once you start redefining what apple pie is, you open it up to ongoing redefinition forever. It's a Rubicon; once you cross it, it's crossed. You can't unring that bell.
The fans of apple pie made with pumpkins pooh-pooh that notion, insisting that this one redefinition is all that will happen. It would be crazy to let any further redefinition of apple pie take place, they solemnly swear.
So we end up with two kinds of apple pie. Made with apples and made with pumpkins.
And finally, someone else comes along and wants to use grapes instead of apples or pumpkins.
The people who prefer the traditional recipe object, saying "Apple pie is traditionally made with apples. Making it with something else is not apple pie." And the people who want to make it with grapes respond, saying, "You didn't stop it when apple pie was made with pumpkins. So you can't use that argument now. Too late. I'm going to make it with grapes, call it apple pie, and you're just going to have to accept it."
Once that redefinition takes place (as it has in Canada apparently), then the door is opened. Those who said this would happen were right. Those who said it would not happen now don't want to think about it; they want to change the subject or argue that polygamous marriage isn't such a bad idea after all.
And if someone suggests that this will lead to further redefinition of marriage yet again, they once again claim that no, it won't, that would be crazy talk. Yet here we are; everything those of us who were against the redefinition of marriage said would happen has happened.
That's the nature of making exceptions. Once you make the first one, you can no longer refuse to make exceptions on the basis that there are no exceptions. Clearly there are, once the first exception has been made. The argument then is only over which exceptions are OK and which are not OK, not whether the exception can be made at all.
"No exceptions" is a strong argument against change until you make the first exception. Then it's worth nothing. Some of us said this; we were laughed at. We were right.