As recently as 40 years ago, interracial couples could not marry. That kind of ignorance and racism is a horrible blemish on our nation's history. Today, we shudder at such injustice and cannot imagine condoning such a law. Denying homosexuals the right to marry those they love signifies a harsh parallel to that unenlightened time of our past.
the burden of proof is on opponents to gay adoption/marriage/homosexuality in general. american society is founded on the notion of negative liberty (as posited by John Locke), which states basically that the limits of one person's freedoms is only defined by how those freedoms infringe (or not) on other peoples' freedoms. gays to not have to justify what they want or do; conservatives must give good reasons why they should not be allowed to. religion is not a suitable basis for such a reason, because religious doctrine does not direct law in america (so sayeth the separation of church and state). beyond that, there is no scientific (social, physiological, or otherwise) evidence that supports any claim that there is anything wrong with homosexuality, including gays rights to adoption and marriage. when it comes down to it, one always finds conservatives' arguments are predicated not on any logical thought process, but rather on prejudice (disgust, fear, discomfort, whatever). It always comes back to that. All of their logic is specious. they start with a prejudice, and then just search for arguments that superficially support it. that is no way to pursue truth, or a path towards a better society.
So...in reference to marriage specifically, marriage is actually not a bond under God (not necessarily anyways, and certainly not for atheists/agnostics). Rather it is a legal contract recognized by the American courts. Since the judicial process should be separate from religious doctrine, why should a religious position have any bearing on that civil process? Besides, not all religions even oppose gay marriage or homosexuality in general. When the Mormon Church tried to represent all of religion against the gay activists in Hawaii, a Buddhist sect stepped in and reminded them it has more members then them in Hawaii, and has no problem with same-*** marriage.
Finally, the notion that it is unnatural is fallacious. Roughly 1/3 of known species in the world exhibit gay/bisexual behavior, including same-*** coupling for life. Ask an animal behaviorist.