It doesn't, anybody can marry as long as they meet the marriage requirements of the states and localities. To claim differently is simply wrong. Ergo, no discrimination.
So let's presume this; in order to get the same civil benefits, homosexuals marry for convenience. Now who who you like living next to you, a quiet, loving couple, going to work each day, cooking out in the back yard, mowing the lawn, you know, normal people stuff OR a nominally married gay couple, hitting the bars at night looking for love/sex, different men/women over every night, and loud raucous parties? I would prefer the former.