The idea that different names don't change the true essence of an object may be valid. But different names do change the way we view that object, or that person, or that institution... And the way others view us can change our lives. This is because we humans are fragile. We wish to belong, to be accepted, to be loved for who we are. If we are separated from the crowd, we feel hurt. Degraded. Lost.
This is why the word "marriage" is so important.
Some say that a civil union is equal to a marriage. Economically, perhaps this could be so, assuming people entering into civil unions were granted the same rights as married couples. But socially? Truly? Separate but equal is never equal.
Dignity. Pride. Happiness. This is what's in a name. Why is there any reason for us to deny gay and lesbian couples the use of the world "marriage"? The only difference it would make is their happiness... and why deny them that? According to, yes, Thomas Jefferson himself, the pursuit of happiness is their right.
That is something you can't change, no matter how many words you say...