I seriously doubt that "religion" is the primary reason why people oppose gay marriage. Even if there was nothing written in the bible about gays some people are simply turned off by the idea of same sex relationships and marriages.
Thankfully the U.S. believes in separation of church and state. Therefore this issue comes down to whether or not people support discrimination. It's not illegal for people to be gay and in love, or cohabitate. Those of us who are straight aren't going to suddenly "switch" just because gay marriage is legalize.
According to the Williams Institute review conducted in April 2011, approximately 3.8 % of American adults identify themselves as being LGBT. You'd think it was 30-40% based on the media.
Religion is just a convenient way of saying it's not "me" but God that finds it an abomination therefore I don't support it.
You see similar objections when it comes premarital sex and promiscuity. People will site unplanned pregnancy and STDs as the reason why it's bad but I suspect if we had 100% methods to avoid both those same people would be forced to admit they simply find the idea of it as being "nasty".
In another instance people site God and religion for why they're pro-life and yet they still fornicate even though the bible says it wrong to do.
Religious people do all kinds of things that God opposes. The vast majority simply "cherry pick" which scriptures they're going to follow and which one's they'll ask for forgiveness.
Ecclesiastes 7:20 KJV
"For there is not a just man upon earth, that doeth good, and sinneth not."