You're pretty badly out of date. More than half the country has supported same-sex marriage for some time. Nearly 3/5ths now support it.
http://www.washingtonpost.com/politics/support-for-same-sex-...
http://www.washingtonpost.com/politics/support-for-same-sex-...
Realize that only 17 states have legalized gay marriage, and only three of those states have passed a popular vote (all three of which were in the past 18 months). I am skeptical that the median of this country is really as progressive as you think it is.
Edit: in the three states that passed a popular vote, none were as high as you are claiming the entire country is. Maryland: 52%, Maine: 53%, Washington 54%
It is widely known that the full population is more liberal than the voting population. Why do you think Republicans try to block voter registration and lower turnout?
[0] http://apps.washingtonpost.com/g/page/politics/washington-po...