Why must people bring murder, rape, child molestation, theft, etc. into these debates as if two people getting married comes anywhere close? Here is the difference, and it's a crucial one - back before the 1960 & 70's it was illegal in most states for interracial couples to marry. It was viewed as "perverse", "corrupt" and "an affront to traditional American values". With that said, the black man and the white woman marrying in the next state over did nothing to demean the marriages of people from the same race. It was simply two adults who fell in love and mutually consented to entering into the bonds of marriage. Same with same-sex marriages.
Rape, murder, theft, child molestation, assault - all involve one person acting in a malicious fashion toward another person (or people) against their consent.