Recently I had an lengthy conversation on the subject of gay marriage. Upon further reflection, I realize that a good portion of the pro gay marriage argument rests on the assumption that it is somehow unfair or illegitimate or discriminatory for government to favor one group or behavioral preference over others. In other words, government should not favor the relationships that produce children and nurture them over relationships that biologically can't reproduce - because, well, it just seems wrong. As an aside, I am naturally sympathetic to such arguments insofar as I think the government should be much smaller and whenever possible allow free people to create and maintain the relationships that they want and in the manner that they want, provided we are talking about consensual relationships of course. The irony is that the advocates of gay marriage tend to be the same folks who want an activist government that clearly demonstrates a preference for certain kinds of relations/behaviors over others. For example, gay marriage advocates would also tend to want an activist government to support racial preferences, or to punish businesses that don't provide health insurance, or to become involved in salary decisions, or to determine the size of the carbonated beverage you are allowed to purchase, and so on.
In a nutshell, the progressive position is that it is illegitimate for government to favor one group or behavior over others, unless it is the group or behavior favored by the progressives.