Using biology as an argument for or against changes to rules in marriage is sort of missing the point. In places where homosexuality is not a crime, people will live in whatever relationship they like. What needs to be considered is what legal recognition of marriage means for couples.
If you choose to subscribe to a point of view in which the word "marriage" applies to union between a man and a woman, then there is nothing to stop you from doing that, regardless of what is written in any dictionary or piece of legislation. If you choose to call your same-sex relationship marriage, then the same applies: no one can really stop you. If someone wants to marry a teddy-bear, no harm is done to anyone, and I don't think it's actually even possible to stop them. Nothing physically happens at a wedding to make a marriage real.
Governments are made out of people, and as people, we should be trying to make other people happy.
Whereas in some parts of the world, being in a homosexual relationship full-stop is illegal, and can get you whipped, in other parts of the world, being in a gay relationship is totally legal, but enjoys no actual legal recognition. Then we have places where gay unions do not have exactly the same rules, rights, and regulations as marriage, yet have some sort of legal recognition. In yet other parts of the world, gay marriage is effectively legal, yet goes under a different name. Then, finally, we have places where there is one marriage law, that is gender-neutral.
Fortunately, there is no government that can actually make it illegal to be gay. I think a good first step world-wide would be the eradication of laws that make living in a gay relationship punishable. As I said, we should be trying to make other people happy, and whipping isn't fun. (unless you're into that sort of thing ;P)
That's fairly strait-forward; being gay should not be a crime anywhere on earth. In the places where being gay is not a crime, there isn't anything to stop a couple getting married, really.
The main issue is all the periphery bits of law- what legal recognition of your relationship grants you. If there was no legal recognition of any relationship between people, then there would be no issue. However, there are many, many bits of legislation surrounding marriage. Marriage, in most places, has an impact on many things financial. It also impacts on adoption. But a little less concrete than this, legal recognition of a marriage flows on to societal recognition of a marriage. If one couple has a wedding and signs a legal document, gets a marriage certificate that is recognised by the laws of the land, and another has a ceremony just the same, and signs a personal pledge that has no legal recognition, the difference is not very tangible, but it is still there.
Financial and property rules and regulations that take relationships into consideration can be, and in some cases are, neutral to gender, and neutral to marital status of the relationship. The same can be said of adoption. All these periphery bits of law could be arranged such that legal recognition of marriage afforded no benefit that was not available to any couple. But that still leaves that last, intangible, feeling of recognition. The equality is not complete.
We need to ask, does legal recognition of marriage, not marriage itself, still serve society, specifically, couples? If it does not, then we should remove it, and let people do whatever the hell they want. If it does, then we should extend the recognition to all couples.
Looking at what is happening in the countries of the world, we are not seeing a dissolving of legal recogntion of marriage, but an alteration of it. Even if we sit here and decide that we do not need legal recognition of marriage, it seems that it is a shorter path to gender-neutral law than no law at all.
P.S. - regarding the Guenther vs thc thing:
The movement to grant the vote to both genders, and to all races, was a movement for the equality of individuals. The movement to grant equal legal status to gay relationships is a movement for the equality of couples. looking at it that way makes that whole Guenther vs thc thing irrelevant.