Marriage has its effects on society.  If a majority disapproves others being allowed to marry on the ground that it damages society, the personal disapproval of one in the majority is quite relevant.  It doesn't answer one bit to tell those who oppose gay marriage, "Well, don't enter gay marriage."  That misses the objection,  In fact, it misses the objection in a snarky, ludicrous, NON-SERIOUS way.