I hate to put it this way,but all evidence from human history, and I have yet to find a single exception, noble ideals aside:
Humans, unless motivated by a personal self interest otherwise (and this includes "feeling good by doing good" motivations as self interest), are naturally selfish, mean bigoted creatures, who find every possible justification to abuse their fellow man for personal gain This is a very negative view of humanity, and humanity has done some incredibly noble things---but I have yet to find one example of nobility that wasn't ultimately motivated by self interest. I would love to be proven wrong on this...
Now, why am I making this point? Because it is the underlying reason for opposition to gay marriage. Marriage between a man and a woman has a number of well documented and established benefits, legal, financial, and social. If those benefits did not exist, then it is unlikely people would have ever established the institution of marriage. Why? , because getting married automatically restricts some previously held personal freedoms, and because human society, to support the biological need of procreation for self preservation, established social moors and values and privileges on people to encourage the procreation, care, and development of future generations.
These moors developed into religious dogma over time, infusing the institution with a magical quality which elevated those int the institution above those not in the institution, thus encouraging more people to participate. Eventually, the religious dogma became infused in the laws, and propagated the cycle and benefit of marriage, so that people would be willing to give up the personal freedoms that marriage removes.
These are SPECIAL rights, legally, applied to ONE class of people. By the US constitution, this violates the 14th amendment as written. Legal and political scholars know this, which is why those who have a vested interest in keeping these special rights, special, maintaining the status quo, are working so actively to change the constitution, and fighting so actively from keeping this from the Supreme court of the US--they don't want the Supreme court to have to find a way around the equal protection clause, because that change can be used as precedent in OTHER cases where they could potentially lose rights and power.
It is MUCH easier to change the constitution to apply selective prejudice against a singe group, to maintain privilege. And if you don't think this would happen, you need to check your US history, especially the various Supreme court decisions over the years--it has happened many times, and the Supreme court routinely reverses decisions based on what is politically popular at the given moment.
Civil unions are a case of "separate but equal", and we have MANY examples in our history that prove that the rights and laws are NEVER applied equally. It comes down to motivated self interest in keeping one group less than you, so that you are more powerful and privileged. Jim Crow, are you listening? Why don't straight people admit that they just want to be bigots and keep their special privileges? Simple--people want to believe they are actually better than they actually are, and thus they have to justify their bigotry and selfishness and hatred with some dogma that makes their vile SEEM to be noble and properly motivated. "Oh, faggots shouldn;t be allowed to marry, ebcause they are UNNATURAL and GOD HATES FAGS and therefore are NOT real people and citizens and therefore don't deserve the right to marry or have homes or even LIVE--- if I had my way about it---it's just the good CHRISTIAN way of looking at it, can't you see that?" Same basic things were said, and justified, of every oppressed group in history. You want to try and justify thinking and believing and acting like Hitler, go ahead. You aren't fooling anybody who isn't also trying to justify their actions.
Would changing the wording to gender, and civil union, and all that, make it more palatable? Probably--it promotes the attitude that "THEY are different, and therefore, not as good or worthy as I am..", and will help keep separate but equal "God damned BETTER keep your faggoty ass separate and definitely NOT equal to my superior NORMAL HEALTHY GOD LOVES ONLY THE GOOD CHRISTIAN STRAIGHT MAN self...". You can all try and justify yourselves as better and more noble than you are, but face it---we are still pretty much monkeys in the trees throwing shit at each other....