Reply #12: "Progressive" simply put means tending toward progress for the human condition.

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It is pro-modernity, science, health, equality, egalitarian, standardization and democracy in the sense of popular accountability. It is against superstition, elitism, ideology, privilege, absolute property rules, oppressive traditions and democracy in the sense people are self-regulating rather than standardized. What I mean by this is that before say 1880, literally anyone could call himself a doctor. There was no standard training and very little hands-on experience. It was democratic in the sense that anyone could do it. The same way with patent medicine, a lack of education standards, a lack of environmental standards, etc. The Progressive point of view is that health care, safety, environmental, educational and just about anything that effects the community as a whole should be standardized based on scientific principles.

To answer the question in your subject line, without any real public policy reason to oppose same-sex marriage, I don't see how someone can be progressive who does oppose it. The objection is purely traditional and religious and has no basis in rationality.