It's beyond "don't care about the gays". I saw plenty of batsh!t crazy liberals on facebook who are not gay (and do not want to get married, themselves) who are adamant in their support for same sex marriage. Now why are they such STRONG proponents for this change to the institution of marriage when they themselves shrug and say "not for me!".
Supporting gays comes from a rejection of religion, it is the main driving force, they know it’s the easiest way to poke the eyes of religious fundamentalists.