I've heard same-sex marriage advocates describe marriage as building a life together, sharing love, sharing commitment, raising children, etc. While all these things are good and admirable, they cannot serve as the only definition of marriage or of a civil union equated with marriage.
Marriage is recognized and incentivized by our society because it provides tangible benefits to society. And chief among these benefits is the creation of new members of society in a committed environment. It is because of this that marriage can be considered a true building block of society.
Consider that a brother and sister, or a grown woman and her elderly mother, or even a whole group of people could build a life together, share love, share commitment, create children by means of medical procedures and provide a safe place to raise them. But we don't propose to extend marriage or its civil benefits and recognition to these and any number of other living arrangements. Why is that?
It's because we realize that the sexual union is a critical component of what we're trying to recognize and protect. And why should the government and society be interested in recognizing and protecting a sexual union? Because the committed sexual union of marriage is the component specifically ordered toward the building up of society by the creation of new members – children.
Some may assert that new members of society can be created in same-sex relationships with the help of technology. But that's kind of the point: In reality, those children are created in a lab by a third party wearing a white coat and goggles, not in the sexual embrace of a marriage. This feat of technology is completely independent of the individuals requesting it (other than perhaps some raw biological material) and is not accomplished or enabled by their relationship with one another.
Now of course there are particular heterosexual couples that are not capable of having children and eventually people progress past their child-bearing years. But society recognizes the type of sexual relationship between man and woman as the one being capable of providing this benefit to society. A homosexual relationship is not ordered toward this benefit for society because of the lack of sexual complementarity on a biological level. Homosexual acts are fundamentally different from heterosexual acts because of these biological differences. So these two types of relationships and the benefits they produce for society are distinctly different and cannot not be equated even on a sociological level.
Equality of Persons
Those who advocate same-sex marriage assert that all people must have equal rights. So let's consider what we've discussed. Heterosexual and homosexual behavior are not biologically equivalent and cannot be equated even on a sociological level.
Individuals, regardless of their sexual orientation, should not have the ability to civilly marry a person of the same sex because that is in reality a type of behavior and relationship different than marriage.
This distinction between behaviors still preserves the same legal rights for everyone, across the board, regardless of orientation.
Now some will say that's unfair. They will say everyone should have the right to marry the person they love. But I must bring up some of the uncomfortable realities of the world. Some people fall in love with close relatives. Others apparently fall in love with children. Still others may fall in love with more than one individual at the same time. We cannot claim that all of these people have a right to have their relationships equated with marriage out of a mistaken sense of equality.
All men are created equal, according to the Declaration of Independence, not all behaviors and relationships.