I've steered clear of this thread for a while, but thought I'd throw in my $.02. Call me whatever you want, but I am totally fine with gender specific groups/clubs (I'll get to it later, but that's what I consider marriage to be - a gender specific group). I'm totally fine with "women only" gyms, and I'm fine with "gentleman only" golf courses (stay strong Augusta). Boy Scouts are for boys, which is why I'm sure one day a girl wanted to be a boy scout, but was told no, and thus we have the creation of Girl Scouts.
I see no problem with two men having the exact same civil rights as me and my wife, but I would prefer that it be called something besides marriage. Marriage for thousands of years has always been between a man and a woman. I hope this doesn't come across as offensive (which usually means it's going to be offensive), but I consider marriage to be our thing. By our, I mean man/woman.
The big problem I see if eventually the definition of marriage is changed, is that genders will be obsolete. I believe in God and I believe that a person's gender is of eternal significance. However, if kids grow up learning that it doesn't matter what gender you are, in essence if gender is subjective - if boys wish to wear dresses, let them wear dresses - then eventually men will cease to be masuline and women will cease to be feminine - and there will be some major sexual confusion.
I have more thoughts, but here's the gist of my manifesto