Adam Seagrave explains why the state is in the marriage business:
Governments don’t legally recognize a certain type of relationship because they are suckers for romance; they do so because they are understandably afraid of the potentially destructive consequences of such romance.
In the now-famous interview with NBC’s “Meet the Press” that precipitated President Obama’s public endorsement of same-sex marriage, Vice President Joe Biden asserted that “all marriages, at their root, are about” the following question: “Who do you love?” As contentious as the recent marriage debates often have been, both the advocates and the opponents of same-sex marriage might agree with the basic point of Biden’s assessment in affirming that something about marriage is more fundamental than politics. Whether this something is a basic right to marry the person of one’s choosing or the traditional institution of conjugal-procreative marriage, both sides agree that there is more to marriage than tax breaks and other legal trappings.
... Although civil marriage is now commonly understood in the elevated terms characteristic of marriage’s more fundamental and profoundly fulfilling aspects, the purpose of civil marriage is, in fact, more in keeping with its sterile legality. Governments assign legal responsibilities and benefits to marriage, rather than to other relationships, to help mitigate the potentially destructive and tragic consequences of irresponsible procreation. -- Public Discourse