The Boston Globe has a smart op ed pointing out the similarities between how the Republican Party is using LGBT issues now and the way that it used racial doublespeak to solidify its support amongst white evangelicals and appeal to social conservatives.
Opposition to gay marriage, along with other forms of gay rights that emphasize the equality of gay and straight relationships, is a key point of connection between Republicans and voters who might otherwise oppose the GOP agenda.
In recent years, gay issues have functioned to help build a Republican coalition in a way similar to the role once played by race issues. In the '70s, '80s, and '90s, the national GOP came out strongly against programs to create special opportunities for racial minorities, thereby gaining substantial support among white social conservatives.
Democrats often charged that the Republicans' opposition to busing for school desegregation and affirmative action hiring plans was a code for much deeper resistance to minority advancement. Democrats saw it as a way for Republicans to signal to voters who opposed voting rights and desegregation that Republicans wouldn't pursue those goals very aggressively.