If gay marrage is ratified how will that effect churches?

Its only a matter of time before gays will start suing church's for "discriminating" against them on the base of the sexual orientation if they refuse to conduct a "marriage" ceremony. It seems, once the "state" recognizes gay marriage as legally legitimate what will stop the gay lobby and the ACLU from taking the next step and start civil-rights class action-suites against Church's who refuse to marry them?