This issue isn't that hard to solve. I as an atheist am often told that I can not possibly have any sense of absolute morality because I have no guidance. First of all: We don't want or need an absolute morality. We want a morality that is thought out, reasoned, discussed, argued and (no pun intended) based on actual intelligent design. In a sense of a non-absolute morality we can say that morality can correspond to the well-being and general happiness of a creature or creatures. In that way moral values can be seen as facts or something close to it.
Now lets look at the gay-marriage issue. A gay man will in fact be happier if gay marriage is legalized. It means the state recognizes his ''kind'' and has the will to do something for them. A gay man is no less than a heterosexual man. If you look at it from the gay man's perspective you'll see that the happiness and general well-being will improve for homosexual people.
Now lets look at the heterosexual standpoint. Will the well-being or the happiness of any heterosexual on earth decrease if this will be legalized? The simple answer is no. There is no possible way in which 2 males or 2 females being married will affect your life. Your well-being and happiness will stay the same.
In fact, any holy scripture is an invalid piece of modern morality. To find the bits in those scriptures that are actually acceptable in modern morality you'd have to cherry pick. Things like not believing in slavery anymore, equality of women, being kind to animals are all recent moral values. They have very little basis in biblical or quranic scripture. They are things that have developed over historical time through a combination of reasoning, sober discussion, argument, legal theory, political and moral philosophy. In those scriptures however there is stoning, killing, oppression of women and homosexuals and even ethnic cleansing. And many people will say ''well, we don't believe that anymore, we've grown out of that''. Well of course you've grown out of it! You've grown out of it because of secular, moral philosophy and rational discussion.
I'm sorry to say that the anti-gay marriage people are 99% building their opinions on biblical scriptures of ''traditional family values''(no such thing exists, but ok) and the traditional form of marriage (which is rather a bond between two people than a bond between a man and a women).
We've abandoned inequality to women, we've abandoned stoning for adultery, we've abandoned death for blasphemy and breaking the Sabbath. Why can't we abandon this in the same way? There is absolutely no person who can oppose gay marriage in a sense of ''good'' morality. I'm not talking about the people who are neutral on this issue. I'm just destroying the myth that opposing gay marriage is somehow ''moral''. Every human is equal, man or woman, black or white,
straight or gay.
Same species, same rights.
On a side note:
studies have shown that homosexual people are better at childrearing than heterosexual people