As for the religious/secular divide, well, someone over on Secular Right is currently hashing out a (secular conservative) case against gay marriage, which is pretty interesting if ultimately (in my opinion) astoundingly unconvincing.
http://secularright.org/wordpress/?p=1940
Hmm...
#1: Yes the majority has rights too; I don't see any trampling or inconvenience here, though. When you need to offer 6 languages, yes it reduces the quality of service you can give to the majority. When First Nation's People are declared to be legal owners of large sections of Canadian cities for historical reasons which are currently inhabited by white people, yeah, that tramples on the white people's rights. When a small contingent of Jews in 1950 decided they could march into the Middle East and force out some Arabs to build a new country, yeah, they're trampling on majority rights.
But gay marriage? Uhh...I don't see how heterosexual marriage is really affected negatively.
#2: Historical costantness--others covered this.
#3: Slipery slope.
* Ponies cannot legally sign a contract (nor can people under the age of 18).
* Incest...well the argument against Incestual marriage is that the resulting kids will be screwed up (the general idea in law being that marriage and sex are at least somewhat linked--see, for example, AIDS tests being required for a marriage certificate in some states). So...that argument could be used to block calls for incest marriage; well...I guess homosexual incest marriage would be okay.
* Polygamy...on paper there's nothing wrong with it. In practice, it seem to be associated with horrible exploitation 90% of the time. That's enough of a justification for laws--see seatbelt laws, for example. People who don't wear seatbelts tend to get screwed up a whole lot worse, though on paper if everyone was a perfect driver it'd be silly to require seatbelts.
#4: You're going to confuse your (apparently) incredibly idiotic populace with all this confusing "change" stuff.
Yeah, uhh...both GW Bush and Obama won their elections with "Change" as a key campaign slogan. I mean, yeah, just from the perspective of a game designer I totally understand the "keep it simple" mantra, and chant it a lot myself. I also understand the concept of "if all else is equal, give players what they have grown to expect." However, gay marriage is not remotely complicated, and doesn't change how the majority of players "play" the game (heterosexual couples still usually get married in a church and sign the same documents).
#5: Human Nature--others have already argued that this isn't human nature but rather church doctrine. Personally I'm not sure it matters even if there is some human nature there--I mean, there are people with piercings that I'm sure make most of society very uncomfortable, and which probably make for weird sex. I wouldn't bar these people from getting married.
#6: This is an age-old one--"why not have some civil union with all the same rights, and just not call it marriage." Much better authors than I have written about this, like my twin (evil twin? Good twin? Whatever--mathematician girl who runs websites):
Why Separate Is Not Equal
California's domestic partnership law grants the same rights to domestic partners as to surviving spouses except for narrow areas BUT, the state agency overseeing crematories does not accept this and refuses to grant cremation licenses to crematoriums on the authority of a surviving domestic partner.
Unless EVERY existing law is changed or countless court battles are fought to establish that "same" means "same" by precedent, some asshat somewhere is going to insist that a law that says spouse does not include domestic partners.
This is the situation I am now in regarding the remains of my partner, Jeanne who died last Saturday. I don't have time to fight a legal battle because the law gives me only eight days to take charge of things and then the county is in charge.
Separate is NEVER equal.
- Joyce Melton
What finally happened is that Jeanne's sister took over and insisted that the death certificate be filled out in Jeanne's original name with Jeanne as an AKA. After 21 years, I had no spousal rights. Except that I had already paid for everything. Jeanne's sister simply took a last spiteful strike against Jeanne.
Separate is NEVER equal.
Hugs to all,
Erin