Libertarians are often described (and some, including the current Libertarian Party nominee, Gary Johnson) as being fiscally conservative and socially liberal. What is implicitly meant is that on economics they tend to agree more with Republicans and on social issues they tend to agree more with Democrats. This is often stated for political reasons, but I think it fall so short to correctly explain the differences between how a classical liberal (libertarian) would address social issues, as opposed to how a Progressive Democrat might do so.
Social liberalism is primarily about the increase of the liberty of the individual. Thus, greater liberty for lgbt, women, drug users, etc. Social progressivism, on the other hand, is less about increasing the liberty of the individual, and more about the progression in societal mores that is brought about through inclusion of previously disenfranchised, or oppressed, groups. In the former, progress is seen through the increase of liberty and autonomy for the individual, but in the latter, progress is measured in terms of the acceptance of the lifestyles of the various subgroups by society as a whole.
This can be seen in the current debates that play out in today’s politics. For libertarians, there is much less emphasis on attributing ill motives to those who may disagree with them, for instance, on gay marriage or transgender rights, as opposed to progressives who see those conservatives who disagree as bigots and people who are holding society back. Progress is not about increased freedom, for that would include the freedom of those who disagree to also lead their lives as they best see fit, even if that includes attitudes and behaviors that display their disagreement with progressive opinions. This leaves the libertarian in the position of saying that whether or not acceptance of new groups is moral progress up to the judgment of individuals and groups, while maintaining that there is at least progress in greater liberty. For progressives, gay marriage is progress in morals, leaving them in the position of judge over those who disagree with them. For libertarians, gay marriage is progress in liberty, and whether or not this is progress in morals is up to individuals and groups to decide for themselves.
A related can contemporary example might fit even better. A conservative Christian baker has been by a gay couple to make them a wedding cake for their nuptials. For the baker, this would be to support something that he considers to be a sin and thus refuses to bake the cake for the wedding. Let’s look at how a social liberal and a social progressive might respond in this situation. A social liberal would take the position that for the government to force the baker to violate his conscience is to impinge on his individual freedom and autonomy. The social liberal may think the baker is being a bigot, but that is besides the point. No one should be forced to violate his conscience as long as his conscience isn’t advising him or her to violate the rights of somebody else. However, the social progressive would assert that the baker is being bigoted and discriminating against the gay couple and must be coerced into serving them, regardless of whether or not his individual freedom and autonomy are violated. For the progressive what is important is the progression in the morals of society, and the baker represents a step backward.
Identifying discrimination is significant because I think that is the key fundamental moral of progressives. Discrimination and oppression are the lens through which key issues are viewed. A world with less of each is a better world to live in. For the liberal, these are both secondary to the key issue of individual liberty. It is through individual liberty that moral progress is made because the good is determined through the free interaction of individuals and groups. Different ways of living together are tried and tested. We reason together and discuss issues and because not everyone will agree, we have to tolerate the differences without being coerced into accepting those differences we may not agree with. Society is thus able to morally “progress” in such a way as society finds certain ways of living to be what works best and those who dissent are able to live out there way of living too. Which leaves open the possibility that the dissenters themselves may find there way living eventually adopted by most people should it be seen as the most desirable. Therefore, tolerance is one of the key virtues of the liberal society, and what distinguishes social liberals from social progressives.