Furthermore, most of the "civil rights successes of the last sixty years" have to do with race, and race is not an issue of Christianity for any but a very few loonies.
Who do you think were the biggest opponents against civil rights issues for non-whites during the mid 20th century?
The same rhetoric about gay marriage was espoused back then, only it was targeting interracial marriage. Verses from the bible were even used to justify it as a "sin".
Christians, on the whole, have probably come to accept the idea that non-whites should be treated equally under the law, but that wasn't the majority opinion back in the 40's or earlier.
In 50 years it's not inconceivable to imagine that the majority of Christians will accept that gay and bisexual people should be treated the same as straight people.