While South Carolina [1] would never be called a gay-friendly state, don’t believe everything you hear. Charleston [2] and Columbia [3], in particular, are quite accepting of homosexuals, and gay and lesbian travelers shouldn’t expect anything untoward to happen. Because of its general religious conservatism, the Upstate could be considered the least gay-friendly portion of the state.
In small towns all over South Carolina, the best approach is to simply observe dominant Southern mores for anyone here, gay or straight. In a nutshell, that means keep public displays of affection and politics to a minimum. Southerners in general have a low opinion of anyone who flagrantly espouses a viewpoint too obviously or loudly.
Links:
[1] http://www.moon.com/destinations/south-carolina
[2] http://www.moon.com/destinations/charleston-savannah/charleston
[3] http://www.moon.com/destinations/south-carolina/columbia-and-the-midlands/columbia