Marriage has never stopped anyone who wanted lead a promiscuous lifestyle from doing so, and it never will. To assume that because gays would have access to government-sanctioned marriage some would not also be promiscuous is to fail to understand human nature.
This entirely manufactured issue of "gay marriage" is not about civil rights, nor about helping people avoid promiscuity. It is just another front in the Left's roughly 150-year-old assault on traditional social mores, and further their mission to increase the scope and reach of big government.