Why not? What's the difference? Lots of people want to marry a first cousin, some would marry half sibling, and a small sector would even Marry a full blooded sibling. If we redefine marriage to include gay couples, why exclude the others.
Yeru, I think that's called a "weak analogy", or is that "poisoning the well"? anyway, I smell bad logic here. Sorry man, I call it like I see it.