Controversial statement time! Everybody ready?
While the development of gay culture was crucial to the public acceptance of homosexuality in the twentieth century, here in the twenty-first, the conflation and confusion between gay culture and homosexuality in the public consciousness is the number-one impediment to conservative acceptance of homosexual marriage.
Here, I'll show you what I'm talking about: "I'm going to grab a couple of the guys and head out to the local gay ______."
What word did you put in there? Church? Gym? Farm? Factory? Volunteer fire department?
No. You thought bar.
When John and Jane Vanilla hear the word "gay", they think of pink feather boas, silly girly voices, skimpy clothing, and the song "It's Raining Men". They do not think of religion, health, agriculture, productivity or an altruistic approach to fire safety. Gay culture is fundamentally hedonistic.
And, as long as homosexuality and gay culture are thought of as not merely statistically correlated, but as the same thing, that will be a far stronger barrier to the acceptance of homosexual union than a mere two verses in Leviticus and one in Romans.