I was thinking about this a little bit yesterday as I drove through west L.A.
I voted that the prohibition is because it contradicts the commandment to multiply and replenish the earth.
But it's not very logical, I believe, as applied to homosexuals, because the church's current position is that they should remain celibate - thus, they can't "multiply and replenish the earth" anyway. In other words, if they remain celibate, they can't multiply and replenish the earth, but they can remain in full activity. So, marrying another homosexual doesn't have any effect on their likelihood of "breaking" the multiplying and replenishing the earth commandment.