"All animals are equal, but some animals are more equal than others"
The interesting thing to note about discrimination (of any nature, though we are obviously talking about sexuality most explicitly) is that many of the people who claim to be against it are in fact reinforcing it by their own internal preconceptions of how it actually takes place.
In theory, everyone knows how such a thing exists. A smaller, more stigmatized group gets the short end of the stick simply because how they are, or how they act, which is in a way uncommon to that of the larger majority. In modern western societies, explicit sexuality is obviously most often noted in the form of homosexuality and the associated issues. People assume that marriage equality means simply only altering the existing structure for that one issue, but that is simply not the case. They are one discriminated group, but not the only. In fact, the reason they are considered the main discriminated group is actually because LESS people discriminate against them than the groups which get less sympathy. The people who consider them the only entities left out of this issue are doing exactly what they accuse others of doing. That is, being more accepting of what is considered more common and accepted, whilst continuing to shun that which they're less forgiving of.
Yes, that's right. We're discussing taboo love. In other words, the most discriminated against of all (legal) relationships. We could bring up polygamy too, if we wanted, but more to the point we're talking about incest. Now I would like to begin by stating that I am not now, nor ever will be in an incestuous relationship. I cannot speak about it firsthand, but in the name of equality know that those who are less will always need those who are more to speak for them. I know that what I may consider an unpalatable love is to some others very real, and consumes a very large portion of their existing love life. That it is alien to me or you does not make it something which is not real.
Now on the topic of incest, many people will try to say “that's different” or that it's not the same as homosexuality in various ways, but the realistic response to that is quite simply “why?” The reason it's “different” is simply because more people remain who are more ready to have knee-jerk revulsion to it, who think the morality of an action is dictated purely by their own preconceived biases which they want to believe come from an objective chem carved into the universe itself, but which in reality are simply fed back into them by the collective of their culture. Incest (between consenting adults, of course) now is what homosexuality itself was a few decades ago. People consider it harmful or odd. Unnatural, and that by extension they believe they have an absolute right to pronounce judgment on it in the most passive aggressive of all ways, to prevent people they have no reason to inhibit from receiving the same governmental benefits that those they believe should be privileged should enjoy.
But to those who still want to declare it “different” and make various excuses relating to the matter such as “But what about children? It's not healthy to reproduce with people who are too close to you genetically.” But so what? Making an argument that they shouldn't or “can't” have children is no different than using that logic to deprive gays or the elderly or infertile with the same rights. Birth control exists for a reason. And even if they choose to anyways, the act of sibling relations in and of itself is certainly not illegal, so why should a civil recognition of it be?
From there you can of course address the science of the matter itself. There are few states that allow marriage between first cousins, even though there are next to no medical issues with cousins reproducing at all, and in fact many cultures simply accept it in general. Which makes us, the “tolerant” west, seem to be viewed in a much different light regarding openness to sexual practices one is unused to. And even moving beyond cousins to sibling, or even twin based relationships, the chance of any kind of a defect between siblings is actually LESS than that of two genetically unrelated individuals having children later in life (for as you get on in years, the chances raise slightly). In fact, problems only crop up when it continues on for generations, feeding back and multiplying people's own genetic weaknesses into their own bloodline. In a world where optimal parents of all kinds can do much harm to a child in various ways, preventing a loving couple from something which has statistically negligible chances to manifest can hardly be construed as anything but another alternate type of discrimination.
And once you get past all those, people's arguments generally devolve back down to “but how many people even want that?!” But how is that even an argument? Some do certainly, many most likely. If there was not such a heavy stigmatization, there would in fact most likely be many more. Which is of course what the issue is in the first place. Why should these people have to hide, be denied, or change their lives simply because others believe they are only a few who can easily be swept under the rug? This is the same thing the oppressors think of every minority. Certainly using the oppressors same logic does not make sense in declaring one's self more egalitarian than them.
This isn't in any way to say that these people have it any worse than gays do now. Many different people have different sets of issues, and have to fight for the ability to be recognized or legitimatized in different ways. But it's important to remember that discrimination is an overpowering concept. Just because someone claims to be against it in one way does not mean they are not still part of it in another. Many people may recognize the ad which shows next to each-other the states gays versus cousins can legally marry in, lamenting over how preposterous it was that cousins had sightly more legality. But this ad design in itself carries with it a rather blatant hint of bigotry; it is not looking at the progress of one group, and lamenting that another is not so far. It is literally feeding off of people's revulsion at one group to gather support for another, while retaining and reinforcing stigmatization for the first. And what's worse, these types of arguments are made by people with no sense of irony to their own actions!
Mind you, none of this is to say that the fight for gay rights is not important. It is. But people are so ready and common to say “gay rights are human rights.” But if that's so, why is it about “gay” marriage rights at all? Rather than chisel away the issues of individual groups people happen to like enough now at once, why not simply go for the main goal. Actual marriage equality for all citizens, for whoever, whenever, why-ever they choose? Egalitarianism isn't a weird word for helping your favorite discriminated against group; it literally means that the goal of society is to move forward holding hands, to help everyone equally at the times and for the purposes they need it. We are all brethren on this one Earth, and we should by now have come to learn that our social issues are all intertwined as well. The world isn't for one, or some, or even most of us; it's for all of us.