Oh boy... this could get ugly.
As Osterley said, marriage has always been the commitment made between a man and a woman. This used to be a religious notion but has since become transformed into a political notion due to the rights and recognitions now associated with a married couple. As such I can understand the need (other than to profess one's love for another human being) to protect ones-self from a legal point of view. For example, to have the same rights during a break-up as a married couple would.
From a Christian point of view, to put it bluntly, same sex relationships are viewed as a sin. I know this isn't a popular opinion now days but there are a number of points in the bible (and no doubt other religious texts) that make non-believers and believers alike uncomfortable as it doesn't fit into 'their way of life'. This being said, it is impossible to live a sinless life regardless of your sexual orientation and all sin is viewed and erased equally through the sacrifice of Jesus, meaning I as a straight man cannot look at a gay man and proclaim that he is more sinful or less worthy of God's love than I.
Whether or not we allow gay marriage won't change the fact that there will still be people in same sex relationships. What it will do, though, is promote same sex relationships as something that is viewed as OK in our society and something that should be encouraged. If we look again at the notion of all sin being equal, this could be compared to legalising theft, or encouraging adultery. While theft and adultery are present in the lives of believers and non believers today, the government / society doesn't generally encourage these activities or make them out to be something that is acceptable.
It's hard to speak about the love of two people not being 'acceptable' the same way theft, murder, adultery etc. are but if sin is viewed equally then the comparison has to be made, even if it is an extremely unpopular opinion in modern day society.
When looking at this topic I do think that above all it is worth remembering that when Jesus was on this Earth he promoted, above almost everything else, the importance of loving one another regardless of who that person is. Many people who held highly regarded positions in the church at the time of Jesus questioned his association and interaction with the leper's, the poor, the prostitutes, the people that were looked down upon by society and those in the church. They couldn't believe that Jesus, the Son of God, would speak to, embrace or heal these 'unholy' and 'un-Godly' people.
I believe that if Jesus was on this earth today that he would continue this message and call all of us to love each other - both gay and straight - with all our hearts and to love them as equally as we love every other human in this world, regardless of their sexual orientation.
As I said at the beginning, all of what is said above is a Christian view. I understand a number of other religions would share similar views and that a number of people on this forum are from different religious background - as well as from no religious background what-so-ever. What I have written is not designed as an attack or to offend but to give a view that is rarely discussed openly in the media or in general conversation. I do believe that everyone should have equal rights when it comes to security and no one should be abused, bullied, hurt or otherwise for loving and being in a relationship with a same-sex partner. I don't think that the only way to achieve this, though, is through legalising gay marriage.