I don't think Christian American Extremists are the reason gay marriage is slow to adapt. Most religions seem to take a stance against gay activity or gay marriage, and the "extreme" southern bible quoters are certainly one of the biggest lobby groups against gay marriage, but I think the biggest obstacle to gay marriage has just always been society's views in general.
Gay sex (in particular man-man gay sex) is just something that most straight people have a natural aversion to. It's a genetic quirk, and like many things that make people different it's feared and rejected. Further, because of it having the added quality of discouraging man-woman sex (and thus the continuation of the race), something in human instinct tells people that it's "bad" or "gross". Our societies thus have rejected gay unions for a very long time.
Today we have pro-gay marriage lobby groups working to make the gay image less strange to society in general, and we have people growing up in a society that is less anti-gay. People have a greater understanding of what gay is. As such more and more people either are cool with gay marriage or just don't care.
It's just a natural society change pushed by pro-gay groups that has happened. I don't think Christian extremists had much of an effect at slowing the acceptance, I just think it's something that naturally takes time.
