I watched the last session of the LDS General Conference. Okay, so I mostly played with my niece, but I did listen to the talks being given in the background.
Dallin H. Oaks's talk really stood out to me. He talked about selfishness, and I couldn't help but think, "That's exactly why I'm probably going to marry a guy."
You see, when I ask why I am supposed to marry a woman (or expect to marry a woman), I am usually told something like: 1) It is your destiny, your heritage, your entitlement; 2) If you don't, you can't get the greatest reward in heaven; and/or 3) It's what God wants you to do.
And when I ask why it's what God wants me to do, it's usually something along the lines of: 1) Because He wants you to return to Him; or 2) He needs you to create physical bodies for His spirit children.
If you listened to Elder Oaks's talk, you'll recognize that he used extremely similar examples to show what he thinks is selfish. He said attitudes such as "I am entitled to [x]; therefore, give me [x]" and "What about my needs? You need to take care of my needs," are self centered.
So, really, when it comes to telling me that I should marry a woman, the Church is telling me that I should be selfish (so I can get what's due me as a son of God) and/or that God is selfish (caring primarily about His needs/desires*).
Now, yes, there is often selfishness involved in deciding to marry someone of the same sex... but no more-so, really, than in deciding to get married at all. That is, a lot of people marry because they are looking for some needs to be met a relationship. This is true in both the gay and straight demographics.
However, in both demographics are also found those who desire to get married to share themselves completely and whole-ly with someone else in order to form a family which family is then used to selflessly raise children to become quality members of society.
I fall into this latter group. And, frankly, in order to give myself completely to someone else -- I'm referring specifically to my heart -- the other person, in most likelihood, will have to be a man.
I would feel and have always felt completely selfish pursuing a woman.
I feel I have a lot to offer someone else, but I know a significant portion of what I can give simply and naturally isn't compatible in the hands of a woman. And when the only arguments that would have me try to make it compatible are rooted in selfishness ("I need to earn exaltation" or "God commands it because it's what He needs"), I have zero motivation to try.
*I actually do not understand the "God needs physical bodies" argument... not with Matthew 3:9 in mind, anyway.