I'm going to let PG do the legwork for me here:

>I suspect the only taboos that are more than taboos are the ones that are universal, or nearly so. Murder for example. But any idea that's considered harmless in a significant percentage of times and places, and yet is taboo in ours, is a good candidate for something we're mistaken about.

Again, from his essay on taboos. [1] Most everything you've cited as an objective wrong was practically universally considered an objective good nary 60 years ago. The morality around slavery was so murky that a years-long civil war was fought between the opposing sides on that issue. Are we just supposed to believe that everyone that lived south of the Mason-Dixon line was naturally insipid, evil, and amoral? As PG states, unless it's universally regarded as evil across all (or nearly all) civilizations, it's very likely that a particular taboo does not cross into the "objectively evil" territory. Slavery has been pretty widespread throughout human history, so that should clue you in that there are possible morally sympathetic readings of it (which usually hinge on the belief that the enslaved group is naturally inferior and couldn't survive without the master group).

>As always, if it were about kids and reproduction, we wouldn't allow infertile and senior couples to marry.

I disagree based on two important elements. Heterosexual couples could become fertile at any time; you never know when infertility will reverse itself if the couple is otherwise healthy and under 40. Post-menopausal women or other permanently sterile heterosexual partners are OK because they are examples that reinforce the need for permanent heterosexual coupling and family structure, even if they are unable to produce children on their own, and secondarily, they can provide a natural parenting context with male-female parental duality, as biologically mandated, if they ever obtain a ward. Homosexual sexual activity can never result in reproduction and can not provide the male-female parental duality that is necessary to produce a child by natural means.

Marriage is really about all of society, and not really about the couple that gets married. It is fine for one to believe that gay marriage is beneficial, but it's not fine to pretend like there is no change in the behavior endorsed and that opposition is based solely on discriminatory motives. Whatever you say you are, or whatever you actually are, heterosexual coupling and homosexual coupling are two different behaviors that could have differing ramifications on society as a whole. Thus, the cost-benefit is worthy of some consideration, and differing opinions are fine, even using a standard that disallows all "discriminatory" rationale (which standard can't really be considered an objective good either).

I'm not really trying to escalate this into a debate on gay marriage, but I think it's important to delineate the logic that gay rights campaigners fight hard to obscure. Gay rights advocates don't want a conversation to get started; they just want everyone to believe that their opponents are naturally evil, so they go around and make a frowny face until it gets people like Brendan Eich kicked out of jobs, and they can then point at Eich and say "He was so evil he got fired! Only evil people would dare oppose us".

Perhaps you don't believe a male-female parental duality is important. Perhaps you don't believe that marriage is an institution that deserves state protection or benefits. Perhaps you don't see any meaning in the evolutionary imperative that children can only be produced by opposite-sex partners. All of this is well and good. You are welcome to your beliefs. The important thing is to accept that others are welcome to their beliefs too, even if they differ from yours, and that that doesn't automatically make them "lesser human beings" (or "bigots", the currently popular shorthand).

>The government should clearly not be subsidizing relationships at all, but since it is, it must do so fairly. Even our insane Supreme Court realized this vis-a-vis DOMA. The 14th amendment is very clear.

The 14th Amendment is anything but "very clear", and again, the fact that there have been so many differing interpretations of it is evidence that its meaning and implications are debatable by reasonable persons. It's a bad, broad law.

[1] http://www.paulgraham.com/say.html