If (or when?) same sex marriage is deemed a constitutional right, how will it affect marriage in general?
Its difficult to say.
From one perspective, its effect will be negligible.
While sometimes it is argued that marriage and family serves as the foundation for civilization, same sex marriage hardly will mean a crack in that foundation. Remember, homosexuals make up only one to two percent of the population. Moreover, despite the rhetoric of advocates of same sex marriage and their fluffers in the mainstream media, most same sex couples will NOT be getting married. While these advocates repeatedly invoke the same sex "committed couple" that have been living together for decades without the benefits of legally recognized marriage, many--perhaps most--same sex relationships are so-called "open relationships." May same sex couples will NOT make those pledges of fidelity etc. under those conditions. (Some still do, of course, even when they know better.)
Consequently, with so few homosexuals in the general population and even fewer wanting the "commitment" of marriage, we really will see little of it. I imagine that even in states that have legalized same sex marriage already, few people actually even know any same sex married households.
From another perspective, the legalization of same sex marriage will erode the status of marriage within our culture by creating yet another domestic family arrangement. We already have plenty of those, thanks largely to heterosexuals. We have traditional married couples, divorced but remarried couples, divorced single head of households, never married single head of households, and heterosexual couples cohabiting without the legal benefits of marriage.
With the decriminalization of homosexuality, we also now have homosexual couples cohabiting. In some states these couples enjoy the benefits of domestic partnerships legislation. We probably soon will add same sex marriage to this smorgasbord of legally sanctioned relationships and living arrangements.
More important, the legalization of same sex marriage will not mean the end of the so-called culture wars over marriage. For limiting marriage only to couples discriminates against bisexuals--those whose sexual disorientation drives them to both sexes.
We have witnessed this already in Europe. The Netherlands and Sweden legalized same sex marriage in 2001 and 2009 respectively. It was thought that this settled the "marriage question." Instead, it moved another type of domestic arrangement to the forefront: gender neutral polyamorous relationships.
Once the definition of marriage is uprooted from biology and reproduction and linked only to the emotional bonds between two ( or more) people, it really is impossible to define it in a way that gives it any meaning at all. That will not stop those who despise traditional social institutions from moving this agenda forward. In their minds, there is always so much more to do.
That's why they call themselves progressives.