Despite regular and repeated efforts to tell us we think otherwise, we--faithful Catholics--do not hate gays. We have just as much love, care, and concern for our gay friends and family as we do for our straight ones (and yes, I do personally have several gay friends and a gay family member, FWIW). We are not fearful. We are not hate mongers. We're not ignorant or on the fringes of some sort of "right-wing" or "backwoods" group. We're not narrow minded, irrational, nor stupid. And we're not worried about what other people do in their bedrooms.
On the contrary, marriage is a public, societal, civic (and for many, religious) institution. Our position is not about denying anyone the right to live and love as they please; it is about the public, civic institution of marriage. And marriage, for us, is even more than that--it's a sacrament, right up there with baptism, the Eucharist, confirmation, ordination, reconciliation, and last anointing. It's in the top 7 things we care about. But regardless of our religious beliefs, marriage, for everyone, is something that spans cultures, time, politics, governments, and with few exceptions (I'm being generous and assuming there have been a few societies, though I don't know of any), it has been recognized as a man and a woman forming a stable bond in which they can raise children.
So that's why we care and are not willing to just stand by and let it be whimsically be redefined by this particular cultural ethos in this particular time. It's too important to experiment with. Democracy is great, but the current will of the majority does not equate to what is true or right. History does not define what is true or right either--it just records what is, so saying "history will prove us right" just doesn't make sense.
It may be that our society continues in the direction it is heading as so many are predicting and that gay marriage will eventually be legalized in every state. That won't make it right, though; it'll just make it what the will of the current majority is, just as the current majority for now seems to still think we shouldn't legalize gay marriage. Two sides of the same coin--democratic majority <> truth, right, or justice in either case.
We are not the ones who made gay marriage into an issue. We did not wake up one day and think "hey, whom can we exclude next?" Our society, like pretty much all societies throughout history (I do have a BA in history and humanities, FWIW), just knew what marriage was and made laws to support it because it is in the best interests of a society to do so. It was they on the other "side" of this issue who have made marriage into an open question and are pursuing us for not agreeing with them. We are simply responding with what we think is true and right and just, upholding what marriage has meant across most human cultures and time. We are not out to get anyone nor actively seeking to exclude anyone. We're just standing by the most time-tested human institution.
I have been accused of using reason to defend prejudice; my motives (and those of all who share the traditional view of marriage) have been questioned. I can understand that--there certainly are those who are motivated strictly by prejudice. And in a sense, you could say that any received/traditional understanding of something is a kind of pre-judgment on a thing, i.e., prejudice. But the problem with prejudice is not prejudice in itself but an adhering to it despite reason--an unwillingness to examine an issue reasonably and be open to change your pre-judgment.
This I and many, many others have done in the case of redefining marriage. Our positions are not based on prejudice but on reason, truth, and justice, which means giving a thing what it is due. Before gay marriage advocates burst on the scene, we (pretty much all of us) just assumed a lot about marriage. It's like you assume your car will just work when you turn it on. You may never think about why or how it works. The same goes for how we think about marriage--it just works and has been proven to work and has existed longer than any other human institution and in basically all human cultures.
So why question it? Why change it? Why redefine it? To make the case for that is tough, and we're saying we're not convinced. It's not about hate; it's not about fear; it's about justice--the chief concern of civil society. It wouldn't be just to grant the same civil benefits to gay unions unless they can be proven to provide the same benefits to our society. This certainly hasn't been proven in any sense of the word I know, and it doesn't even bear out in theory--just using reason to think about it.
The real problem we're facing here--the reason so many people (IMO) don't seem to get us is that marriage is already in a bad way in our society. Soaring divorce rates and the consequent perception of marriage as a very temporary arrangement. The perception that marriage is some kind of love story ending (it's really just the beginning). The perception that marriage is a human right (it's not--it's a privilege). Devaluing of new life in the family, and a basically complete lack of understanding of the meaning and value of sex. The best answer to these problems is to work to fix and strengthen marriage, not to further undermine it by reinforcing these problems through redefining it to include other forms of human relationships.
The American Catholic bishops have recently said most of this, probably better and certainly more concisely than I. You'll forgive me for coming across a little defensive, but it gets old having people telling you and others lies about what you think and feel, especially when it paints you as a terrible caricature and it just plain ain't true.