I just can't understand why people waste so much of their energy to deny people marriage."

It's all about political posturing.

It begins with demonizing a minority group (In this case homosexuals). If you read on, you'll hear about "the homosexual agenda" (poised, ready to, among other things, destroy tradition) the standard "homosexuals are pedophiles," and my personal favorite, "THE HOMOSEXUAL MAFIA."

After the demonization is complete, the next step is to convince the masses this targeted group wants to "take away" from them something they value, in this case, "the sanctity of marriage." This carries the masses from mere fear to a more aggressively defensive mode, highly charged with even more wild accusations. Then come the pleas for money to stop these demons, and ultimately, when the conservatives sense they are going to have low voter turnout for an election, they wave the carrot of "stop the demons" under their constiutent's noses, and entice them to get to the polls. While there, voting to save not only their marriages (which some actually revere and respect, although of course not all, but never mind that minor detail) but also the children of the nation, the flag, religion, the moral fiber of the country, and I'm pretty sure, the right to vote. While the voters are there, in the voting booth, preserving what precious little they have left of their lives, they might as well check a few other boxes too, for those conservative candidates.

And? That's how (some) republicans win elections.
Oh yeah, and how the Rushes and the Becks get the raises they're demanding.

You have a good point, assuming that gay marriage opponents are seeking only to defend their ideals of marriage.
What it really comes down to, though, is normalization. There is a very vocal group of Americans that violently oppose anything short of defining homosexuals as miscreants and a danger to society. This is indicated in the equally violent opposition to civil union legislation, hate crimes legislation, and DADT repeal.
I think the most insidious argument used by this group is the "they'll teach homosexual sex to our cute little children!" argument. This is akin to the eating babies assertions used to demonize so many ethnicities.
I would love to think that people are generally reasonable, and that it is all a misunderstanding about semantics and definitions, but the truth is, many people are still using superstition to justify hatred, a suspicion of the "other." These people cannot be reasoned with, reason is the enemy of faith, and faith of reason.
Edit for post script: I think another reason that we are looking for big legislative changes now is because what we really want is social change, we don't want to have to endure this humiliating public dissection of our personal lives, we want our families to treat us and our partners the same as they treat their heterosexual family members, we don't want to be afraid of violence, or ashamed to hold hands when all the straight people around us are holding hands. Marriage would accelerate social change, though it may also briefly intensify confrontation, but social change comes very slowly. It is very hard to come to terms with the fact that we won't see that kind of social revolution in our lifetime, that we will die without reconciliation with our families, without our partners being considered family, we will die as second class citizens.
We will fight for a better world, but it isn't a world we'll see.