Interracial marriage and gay marriage are not equal. There is nothing wrong with being a different race.
People once used scripture, and some still do, to try to justify their bigotry in terms of interracial marriage. A comparable issue today is gay marriage. La dee dah. You're the same gender. I don't really have a reason that I need to care about that, or get all in a fuss about it.
No. They are just not contributing to the population. But being gay and being a heterosexual couple without kids are not the same thing. A heterosexual couple is still doing what nature intended, despite their lack of procreation.
As far as nature's concerned, your primary function is to ensure that your species continues to exist. Look at some other species. There are numerous insects where the adult males don't even have mouths. Some don't have a functioning digestive system. Their purpose is to mate before they starve to death, die of old age, or are otherwise killed. Or queen termites: They're so large that they can't leave the nest. They exist to produce eggs, keeping the nest populated.
Humans: Look how powerfully motivated we are by hormones. "Rape" is a word that exists for us. It's something in our vocabulary, and it can include violent attacks for the purpose of sexual pleasure, without regard for the victim. To me, that alone shows just how absurdly powerful the sex drive is. (And how little control some people exercise over their own minds.) Look at what things we do to secure a mate. Immense resources are devoted to the cause. People are even willing to kill for it.
Numerous plants and animals
die after producing seeds, eggs, or offspring. Nature's willing to say, "Ok, you've done your job. You can die any time now."
If you're not making sure that the next generation is created, nature doesn't have much purpose for you.
We've arrived at a place where we'd like to do more than that. We value things beyond simple mating and reproduction. Saying that a couple who is not producing children, whether by choice or by biology, is "defective" or "useless" serves only to reaffirm that primitive notion that one's only function of value is to successfully reproduce, thus spitting in the face of many hundreds of thousands of years of evolutionary and cultural progress.
Then why has marriage always involved at least one man and at least one woman since the dawn of time?
Clearly you "common-sense" definition of marriage is not consistent with how people have defined marriage.
EDIT: Ask yourself if your definition of marriage is "correct" then why have incestuous marriages been illegal?
As has been pointed,s lavery existed for a very long time as well. The Old Testament speaks of it, and gives rules on beating slaves, freeing them, and general treatment, rather than saying, "People are not property, and I, your God, am only going to warn you
once. This notification
is your warning."
(Good thing Jesus erased his own Old Testament rules with the New Testament stuff.)
Some traditions can die without being missed.
Other traditions are modified throughout the years. Christmas Tree or Yule Log, anyone?