Sex. Food. What is more basic to what it means to be human? Here are things that born humans KNOW for certain:

1. You have to eat.
2. You have to procreate.

The second is why gay marriage "threatens" people. It is hard-wired into us that a woman desires a man, and a man desires a woman. If a person can be convinced that one of their pillars of reality is questionable (that what they instinctively KNOW to be true is relative), then

EVERYTHING is up for grabs.

That is frightening and unthinkable. That's why gay marriage is a "threat" to marriage--a union that creates life.