No, it's actually very easy to justify rationally: for example, sending young men to war lets you replenish the population faster than if you'd sent everyone equally.

I said it's difficult, not impossible. In times of tribal crisis, yes, men must die first. As The Senior said so eloquently, All societies are based on rules to protect pregnant women and young children and anything else is automatically genocidal. A lot of our biology has been geared towards this because it had to in order for our species to survive.

I'm not saying that sexism is automatically and always bad. I am saying that, in times of relative peace and plenty, when survival is not an issue, the justification for sexism in the eyes of the law is just not there.

We don't lose enough soldiers in war that we really need to worry overmuch about replenishing the population. Even the losses in the Civil War or WWII were a pretty insignificant piece of the population compared with the genocidal battles that were common in antiquity, and still go on today. The government in America has no reason to be sexist.

your concept ignores the both the purpose and the practical workings of marriage all through human history

Sorry, gotta call bullshit on that one. Exclusively heterosexual monogamy is a relatively new concept, as far as human history goes. You don't have to look very far to find polyamory (along with polygyny and polyandry), bisexuality/homosexuality, and non-binary gender roles accepted as norms. Look at feudal Japan, or classical Greece and Rome, or ancient Egypt, or the various native American cultures. Throughout most of human history, the rich have traditionally kept multiple lovers, often of varying genders; by historical standards, we're all pretty damn rich today.

Each of those cultures had their own hang-ups, to be sure. My point is, there's a wide enough spectrum that we should be skeptical of any statement that tries to abstract directly from the mores of the West of the last few centuries to a general statement about "all of human history".

I can imagine elephants inventing marriage, but I can't imagine them accepting gay marriage on the same terms as straight.

I suspect a failure of your imagination, rather than a failure of elephants. Giraffes, dogs, chimps, lions, flamingos, sheep, and rams have all been observed engaging in homosexual behavior. Same-sex penguin couples have been observed pairing for life. I don't know if this has ever been seen in elephants, but as they are a very social and playful species, I'd be surprised if they were any different from other pack mammals and birds in this regard.

Of course, there are a lot of "natural" things that we nonetheless don't allow. Rape and murder, for instance, are pretty common among primates, and humans have been doing this forever. And lots of cultures have had their own weird sexual mores that we've thankfully abandoned. As humans, we get to sort out what makes sense for us, on the basis of individual freedom.

My point is that, if you're going to take the position that committed homosexual pairing is somehow "unnatural", or if you think there's no evolutionary benefit to homosexuality, then You're Doing It Wrong. Evolution is pretty smart. It knows what it's doing.

If you're a kid and you have a gay uncle, and that uncle is in a committed relationship, then that's one set of cousins you don't have to compete with for food and attention. You've basically got an extra set of parents to provide for you.

For example, I have explained how tax breaks for married couples benefit society: they give the provider an extra incentive to get and stay married, thus encouraging good offspring.

Gay couples encourage good offspring as well. Just not their own biological offspring. They're quite likely to adopt, and are additional caretakers for their family's kids. By being married, they're better at this.

Homosexual marriage also benefits society for several of the same reasons that heterosexual marriage does. Married couples are generally more careful with their spending, take better care of their homes, and tend to have a greater investment in their neighborhoods and communities. We ought to encourage that with our communal acceptance and respect, since it benefits us all.

Let's focus on the tax breaks.

Barking up the wrong tree, buddy. I'm against the taxes that marriage provides breaks from. If the government can't get enough by asking for the money, it doesn't have the right to take it by force. The IRS makes less from income tax than we spend on our military empire—one injustice to pay for another.

Just as I would encourage a slave who breaks the law to escape his slavery, I support anyone getting out of whatever taxes they can dodge by any loopholes they can find. More power to ’em.