Alright then.  Because marriage is intended to produce and support children, providing them with a stable and loving home?  Well let's handle the 'producing' part of that first shall we?  If you go by this logic everyone should have a fertility test done before they are married.  If one of the two aren't able to have children, they cannot marry.  This includes homosexuals, but also women who have had hysterectomies or gotten their tubes tied, elderly couples who have passed child rearing age, men who have had vasictomies, men with overly low sperm counts, women who take birth control pills or employ other contraceptives, people with medical conditions that make them unable to have children, or hell, just people who don't want children.  Okay, so now we've decided who can't get married and who can.  What if a couple gets married and decides afterward that they don't want kids?  Are they legally divorced?  After all, they aren't intending to produce children and that's the point of marriage...  Now lets say that they do have kids, raising and supporting them together.  When Mommy cheats on Daddy or Daddy starts beating on Mommy, normally divorce would be an option, but then they wouldn't be giving their kids a stable and loving home would they?  So do they have to stay married?

I know it sounds ridiculous to look at it that way, but that's exactly what it means to ban homosexual marriage.  You can't exclude one group of people with that kind of justification without it starting to apply to EVERYONE, because that's the way laws evolve. 

Religion isn't the point.  

Protecting the institution of marriage isn't the point. 

Making sure that every US citizen has the same rights IS.