As a scientist -
The purpose of reproduction is the continuation of life. A same-sex marriage can not produce new life.
As a citizen -
Marriage is a legal contract between a man, a woman, and society that helps ensure that the children the marriage might produce are given adequate care, protection, and training to become contributing members of the society. Children do best in homes with both a father and a mother, and so society binds a man and woman together by law in order to give their children the best chance possible. Laws that give tax breaks to married people were originally intended to help homes where children are being raised. Same-sex marriages can not produce children, and so I do not believe they should have the same protections as traditional marriages.
You might argue that then by the same token, marriage licenses should not be granted to couples who are infertile. Infertility is a difficult thing to determine. I have more than one friend who was told by their doctor that she and her spouse could not have children but then later she had a child. On the other hand, a same-sex couple can never, never, ever produce a child.
As a Christian -
Marriage is a covenant between a man, a woman, and God by which the man and woman promise to be loyal to one another and to care for the children that God sends them. In the beginning, God created Adam and Eve, man and woman, to be companions for each other. That was His plan from the beginning for the happiness of his children.
As an adult human being -
Sexual feelings are a powerful and important part of human life. But in our modern world when we can so easily cheat nature we tend to forget that the purpose of sexuality is . . . REPRODUCTION. Simple as that. Not just for fun. Not just for a high. Not just for that sense of well being and the wonderful emotional bond it creates with a partner. The purpose of sexuality is to create strong families with a mother and father intimately bound in love and the children brought into the world under that shelter.
Any other use is an abuse. Like taking a prescription drug that hasn't been prescribed.