The concept of marriage has been held as a civil right by the SCOTUS over a dozen times since the late 1800s. Therefore, such precedent allows the topic to be reviewed by the judiciary on such matters. The 14th Amendment provides for protection of the citizens/residents/transients of a state, declaring that they are entitled to the equal protection of the laws of that particular state. Such protection can only be denied if the state can provide a justified reason for doing so. So far, that hasn’t happened. Arguments usually used by those who defend the bans include an appeal to nature, procreation, tradition/history, and slippery slope. However, upon further scrutiny, none of these hold up.
Whether an act is natural or not (exactly how does one define something as “natural”?) doesn’t inherently make it good or not, so the question is irrelevant. However, even if we were to entertain it, it has been found through scientific research that over 1,500 species have been documented to express homosexual behavior. In fact, in some species (still subject to debate), it has been found to be the most common sexual behavior witnessed. But still, the question is useless as a barometer of ethical debate.
Procreation isn’t held as a prerequisite for a marriage license for heterosexual unions (elderly, infertile, by choice, etc.), so it seems selective to hold it up for homosexual unions. Even so, homosexuals have methodologies that they can turn to in order to have children (surrogacy, adoption, etc.) that heterosexuals also use and aren’t seen as a lessened status, so the point is moot.
Just because something is tradition or has been the way of things in history doesn’t make something good or not. Slavery, anti-miscegenation laws, coverture laws, etc. were all products of tradition and of historical precedence for a time, but have all been found to be wrong. Institutions like marriage and what constitutes a relationship have changed many times throughout human history and have found various expressions.
The slippery slope argument, the idea that allowing homosexuals to marry will lead to people marrying family members, multiple people, children, non-human animals, or inanimate objects is perhaps the weakest one of all…not just because it is the only one that doesn’t focus on homosexuals at all. Nothing inherent to homosexuals automatically disposes to such a point, and there is also the fact that each of those groups have their own reasons, irrespective of homosexuality, that are reasoned/used to deny them equal protection: consent, legal hangups, psychological/mental abuse, kinship ties, etc. If such groups ultimately fight in court someday, it will be in destroying said arguments, and that will be difficult to say the least. An argument that one group should be granted rights because another unrelated one was will not hold up at all, just as it didn’t work for homosexuals for a time who fought on the notion that they should be granted their equal rights because African Americans got them.
In time, the spirit of the Constitution continues to move forward, with intermittent steps to be sure. It is a never ending struggle, and hopefully will continue into the future.