Who do you think were the biggest opponents against civil rights issues for non-whites during the mid 20th century?
National Socialists, Fascists, and assorted white supremicists. Whom were you thinking it was?
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".
Acutally no, it wasn't "rhetoric about gay marriage." It was rhetoric about interratial marriage. You should also know (in case you don't) that many African Americans take offense at that comparrison.
So called "gay marriage" is its own topic.
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.
Where do you think the abolitionist (anti-slavery) movement came from, and when? It was from the Christian church, and well before the 1940s.
You seem to be confusing the positions held by a minor portion of some American churches with that of Christianity world-wide. That isn't a good assumption.
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.
They are. God loves people that engage in homosexuality or bisexuality and offers them the same forgiveness of sins in Christ as anybody else.