I am not going to take the anti-gay route, but I am going to go to the point of the question.
Marriage is about finding a life partner. And a life partner generally has unique qualities that attract one another.
For example, some women look for men that are strong for protection, smart because it leads to smart children, emotional and financial stability because it gives them a security that will go on to the children.
This also goes into the opposites attract, and brings out the best in their children having good habits and qualities from their parents.
Now as this question is not directly about the gay agenda, I can support gays getting married with the exact same concept that they are looking for partners to raise a family with and thus will be attracted to certain qualities that can lead to a life partner that will support the family in different aspects.
Ultimately marriage is the first step in creating a family (offspring)
So... If people think marriage is about love, then what is reproduction is about? Is that just dubbed sex?
From a scientific perspective, reproduction > love. Marriage, if it supposed to be a state-supported institution, ought to favor both... But with different names. That could mean I'm married to my best friend. I have love, loyalty, and trust in him/her, but I'm never going to say I'm married to her. So what's the deciding factor? Sexual relationships.
Sex makes a marriage. Sex is for reproduction (and when needed, pleasure). Thus, marriage is about production.
Like the previous comment, this in no way reflects some bizarre anti-gay sentiment that people may falsely accuse this argument of. It's just dealing with wording and debate logic!
If the purpose of marriage was procreation, then infertile couples would be prohibited from marrying. There are fertile couples who get married and remain childless; a sociology professor I had in college is one of them. It is the argument that the purpose of marriage is reproduction that is given as a justification to ban same-sex marriage, which is groundless.
You can have babies without having to have get married.
Similarly, you can be in love and want to show commitment to one's relationship without getting married, but it's what people tend to do if they want to show their true commitment to their relationship.
Of course, arguably, the whole institution of marriage is outdated and should be scrapped. But as it exists, people use it as a way to show their commitment to one another. It doesn't produce babies by getting married. You can be married and be celibate. You can be unmarried and copulate hugely.
Marriage is about love, loyalty, and trust. All 'committed' couples should be extended the same state sponsored benefits.
Dr Simon Crouch is a medical doctor and Honorary Research Fellow with the Jack Brockhoff Child Health and Wellbeing Program at the Melbourne School of Population and Global Health, The University of Melbourne. Dr Crouch suggests that there are five key components that logic and empiricism dictate:
1. Many married heterosexual couples cannot or choose not to have children
2. Many unmarried heterosexual and same-sex couples do have children
3. Marriage and children therefore are not necessarily linked
4. Societal acceptance of same-sex families through equal recognition of same-sex relationships in the form of marriage will play a role in reducing the stigma experienced by LGBTI people and their families, improving health and wellbeing.
5. There is no evidence from anywhere in the world to suggest that marriage equality has a negative impact in any way on those who are already married.
Marriage used to be about support and the protection of each family member. Marriage is a cultural and social aspect, therefore doesn't follow the rules of biology, but sociology. The definition of marriage is "a socially supported union involving two or more individuals in what is regarded as a stable, enduring arrangement based at least in part on a sexual bond of some kind." We've come far in the society we live in, no longer do we need people having as many children as they can. We can focus on the growing relationship and the sexual bond between our lovers.