I am slightly worried to learn that a government license of marriage still carries so much weight in the US. One way of reducing this burden is for companies to start recognizing one significant other for health benefits - irrespective of "marriage" (gay or not). I don't know if this violates existing law. Divorce benefits and inheritance could be worked out on a contractual basis. Well, the State already has lots of marriage related laws so there will be backward compatibility problems. Thus, a part of the progressive agenda can consist of convincing government to withdraw all existing marriage related laws. Today they are biased against gays. But where will that leave polygamists tomorrow?
An alternate approach is to sufficiently politicize the concept of marriage so that you win over a majority of voters over a period of time, as the laws are patently biased. This also restricts the freedom of companies and other associations from acting out their own social views, and stick to just enforcing government statutes. Moreover, you might have just one big battle to fight, perhaps at the Federal or even Supreme Court level.
The thorniest issue is adoption as it involves a 3rd human being, and a child at that. People with progressive views generally place great importance on child rights. Obama, for example, has a whole page on Child Advocacy (PDF), which mentions his "Responsible Fatherhood and Healthy Families Act" (link), wants to "Promote Healthy Lifestyles", and penalize fathers for not paying timely alimony. I don't know if same-sex parenting of a child will be viewed to have any negative/positive impacts.
India has some news on marriage laws too! The Maharashtra government will give alimony rights to women in live-in relationships (pending New Delhi approval). This India Together article gives an example of how the law can help a young, urban, working woman: "In Delhi, no one asks her prying questions like why she isn't married yet or what she does with her huge salary or why she lives in with her boyfriend - who shuttles between Delhi and Mumbai...(She) only dreads her mother's periodic visits to the city. (Her mother) worries about what would become of her if her boyfriend decided to break off with her one day....So, while it may be some more time before legal support for women in long-term live-in relationships across India comes into force, her mother can at least lay some of her fears to rest. "