It is important to understand how after millennia of marriage being defined as only a union of one man and one woman, we now are seeing people attempt to change that definition to accommodate a very small minority. How we as a society arrived at such an action is not all that surprising when you consider what has happened to a once-sacred institution.
No-fault divorces have reduced marriage from a serious decision into one where if someone changes their mind, no matter how trivial the reason, the persons involved can dissolve it. Prenuptial agreements damage marriage even more due to the fact that when two people enter such an agreement, they are already thinking about what will happen in case the union doesn't work out they way they may want.
Marriage was meant to provide a stable environment for the raising of children. Now in pop culture having children out of wedlock is not seen as shameful anymore. And we see that state adoption agencies will give a child to a single person, two men or two women. No one seems to think about the child, only what the adults deem to be "fair."
Once accepted as a sacred institution given to people from a higher power, marriage has been reduced to a business transaction and now a civil rights issue, so it is not surprising that so many of the populace embrace the marriage of same-sex partners. Why not let them get married, people will say -- if they regret it, they can always get out of it easily.