I think the key is to ask how all of this is fair to people who don’t marry or to adults who would like to live together but have no sexual relationship. Why does marriage deserve societal support, anyway?
If marriage is about couples, and not about families, then there isn’t a reason that couples ought to have rights that single people do not have. After all, if couples are taking care of each other, shouldn’t it be single people who get special tax breaks, since they have no one in particular to take care of them?
If marriage is to be worthy of societal status, then, it must be about rearing children, which all of society has an interest in, since these children will grow up to continue civilization. Children, of course, are the fruit of heterosexual unions, not homosexual ones. While it is often argued that homosexual couples could adopt orphans, there are two problems with this argument.
The most obvious is that this is not what most homosexual couples are doing. Instead, they turn to heterosexual reproduction. Unlike heterosexual families broken by divorce, bringing children into a homosexual home requires purposefully choosing that one’s child will have a parent that is excluded from the intimate life of his the biological parent he or she lives with, in favor of a lover preferred by that parent.
The second problem is that the ability of two adults to raise a child has no necessary connection to any sexual relationship between the adults. A convent can love and raise a child; the sisters don’t have to have sex with each other to do that. It is impossible to demonstrate that adults who aren’t in a sexual relationship are incompetent at child-rearing. So, again: why unfairly give homosexuals legal advantages that couples with no sexual relationship don’t get?
So while homosexuals and single people might rightly complain when secular laws continue to give heterosexual couples perks but stop requiring heterosexual couples to be responsible to sole reason their unions deserve societal support at all, that does not imply that homosexuals have some right to get in on an institution started with the intention of serving people doing the work of rearing children but that now hands out perks to adult couples whose only goal in life is to suit themselves.
A civil union, in contrast, is a legal avenue by which adults who may not marry each other can nevertheless direct the state to automatically confer a legal status of mutual care for each other. That does not imply a sexual relationship must exist to give the legal direction a reason for existence. It is not fair that adults who want to care for each other should have to have a sexual relationship in order to have the status that goes with mutual care. Again: it is difficult to argue what interest the state has in encouraging homosexual relationships over celibacy. I don’t think there is a leg to stand on.
Therefore, heterosexual relationships, being inherently fruitful, deserve the support due to the work of childrearing, which is for the common good. Other relationships between adults, while they may have fruitful aspects, cannot possibly have a fruitful sexual component, and therefore cannot claim a right to special priveleges in recognition of a sexual relationship between adults in those relationships.
