Joe Carter in First Things with a must-read on how cultures can be changed by dedicated activists:

In his book The Future of Marriage, David Blankenhorn, a liberal, gay-rights-supporting Democrat and self-professed “marriage nut,” offers this sociological principle: “People who professionally dislike marriage almost always favor gay marriage.” As a corollary, Blankenhorn adds: “Ideas that have long been used to attack marriage are now commonly used to support same-sex marriage.”

Blankenhorn provides almost irrefutable proof that this is the expressed agenda of many—if not most—professional advocates of same-sex marriage.

All issues fall somewhere along this policy continuum, which can be roughly outlined as: Unthinkable, Radical, Acceptable, Sensible, Popular, Policy. When the window moves or expands, ideas can accordingly become more or less politically acceptable [...] If the goal were to undermine cultural institutions [such as marriage], the process for getting from Unthinkable to Policy would follow these five easy steps:
Step #1: From Unthinkable to Radical
Step #2: From Radical to Acceptable
Step #3: From Acceptable to Sensible
Step #4: From Sensible to Popular
Step #5: From Popular to Policy

Of course not everyone in society will agree with every step along the way, but that won’t stop an issue from sliding into policy. All it requires is for a majority of the people who find the issue unacceptable to do nothing at all.