The idea that gay couples cant marry because of the disconnect between homosexuality and procreation cuts across many arguments against gay marriage. Gay marriage would be unnatural because it can't produce children, the natural end of marriage. Gay marriage would undermine marriage because it is a legal and moral institution designed to promote and protect procreation and raising of children. Gay marriage would desecrate God's mandate that heterosexual couples have to mate and procreate. Is any of this true, and if so, does it matter?
Consider the assumption that the natural end of marriage (or sex in general) is procreation, and that therefore non-procreative gay couples cannot reasonably be allowed to marry. There are two ways this can be refuted: by showing what its logical conclusions would be if actively employed, and by taking apart its philosophical basis.
First, if we were to take this premise seriously, we would have to radically change marriage laws. No infertile couples would be allowed to marry this would include both younger people who are infertile due to health issues as well as older people who are infertile due to age. Who would agree to that?
It is curious that the opprobrium heaped upon gays who want to marry is not also directed upon elderly people who want to marry, indicating that the problem cannot possibly stem from peoples disapproval of a couple that wont be having children. Consider peoples reactions when someone gets married for reasons other that love, like citizenship, money, or social status. This indicates that society regards love as the basis for marrying, not producing children.
If we were to enforce the idea that marriage exists for the sake of having and raising children, wouldnt we prohibit couples from remaining childless voluntarily? Even if we didnt outlaw both contraception and abortion, we would have to take steps to ensure that all married couples not be childless: if they wont produce their own kids, they will have to adopt some of the many orphaned and abandon children currently without stable homes and families. Since we dont see anyone arguing for such outrageous measures, we must conclude that opponents of same-sex marriage dont take that principle as seriously as they seem; and because such measures are so outrageous, we have good reason not to take it seriously either.
Gay Couples with Children
Even without those conclusions, the premise itself has a number of flaws. It contains the idea that there is an essential disconnect between homosexuality and children, but this is mistake. Gay couples are not universally childless. Some have children because one or both partners were earlier involved in a heterosexual relationship that produced offspring. Some gay male couples have children because they have made arrangements with someone else to act as a surrogate mother. Some lesbian couples have children because they used artificial insemination. Finally, some gay couples have children because they have adopted.
Whatever the reason, more and more gay couples are not childless and if marriage, whether in nature or as a legal institution, exists to promote and protect both procreation and the raising of children, then why cant it do so for gay couples as well as straight couples?
Biology and the Sacred
A second flaw is that it makes a fetish out of biological functions. Since when do people tailor their activities based solely or even primarily upon what they imagine the biological ends to be? Who gets married solely to have children and not to pursue a meaningful and intimate relationship with someone they love? Who eats food solely in order to ingest nutrition and not to enjoy the social and psychological experiences that accompany a good meal?
Finally, it is argued that the existence of gay marriages would constitute a desecration of a sacred institution created by God for the purpose procreation. This might be true if churches that regarded homosexuality as an abomination were forced to perform and recognize same-sex marriages, but no one is suggesting this occur.
Civil marriages, established and regulated by secular laws in a pluralistic society, cannot be restricted by how some religions conceive of marriage from within the theological boundaries of their faith. Marriage between members of different religions cannot be legally proscribed simply because some churches consider it sacrilege. Marriage between members of different races cannot be legally proscribed simply because some groups regard miscegenation as contrary to Gods will. So why should marriage between members of the same sex be any different?