According to The Book of Common Prayer,
The union of husband and wife in heart, body, and mind is intended by God for their mutual joy; for the help and comfort given one another in prosperity and adversity; and, when it is God’s will, for the procreation of children and their nurture in the knowledge and love of the Lord.
So, marriage serves multiple, partly overlapping functions:
–Personal happiness and fulfillment (including sexual pleasure!)
–Companionship and support in facing the joys and vicissitudes of life
–A context for the having and raising of children
Even though the BCP refers to a “union of husband and wife,” it seems obvious that same-sex couples also desire–and benefit from–the goods that marriage provides. And individual marriages will exhibit these goods in different combinations and to varying degrees. There are straight couples who don’t (or can’t) have children and perhaps don’t derive much happiness from their marriages, but who marry for support or economic security. Likewise, there are gay couples who have children, either from previous marriages, by adoption, or through assisted reproductive technologies.
That’s why I think people who suggest we have a “two-tiered” marriage system–one for opposite-sex and one for same-sex couples–are making things overly complicated. Marriage is big enough to accommodate a variety of different relationships. What would be the point of establishing separate, parallel versions of marriage when the existing institution is already flexible enough to accommodate same-sex couples? (As it already accommodates infertile or elderly couples, say.)
What proponents of such a system sometimes say is that we need to preserve a “straights-only” version of marriage to uphold the value of “sexual difference.” Now, you don’t need to buy into what some Christians call “complementarianism” to acknowledge that there are differences between men and women; if nothing else, their different biological natures are what make human procreation possible. But I don’t understand what purpose is served by setting aside a special institution just to express this distinction. Providing meaningful social support to people raising children strikes me as far more important than symbolically emphasizing the specialness of sexual difference.