Except for a couple of facts. First of all, you can kill whoever you choose to if you actually are capable of doing it, but that doesn't mean that you can guarantee that you will not face some form of punishment, either from society or from the family/friends of the deceased. On top of that, I highly doubt that God would be very happy with you killing a person just because they were a man who wanted to marry another man or a woman who wanted to marry another woman. Last time I looked, that was definitely not what Jesus would do.
Secondly, I believe that most people have certain agreements on certain right and wrong things. This generally leads to what becomes laws for a society.
The thing about these arguments is that some of us base laws off of the morality that is set down in the US Constitution, which states that every person should have the right to life, liberty and the pursuit of happiness. By this phrase is how I base my interpretation of what laws should impose upon the population. It requires a very simple test. Do the actions of a person or group affect someone else's right to life, liberty, or the pursuit of happiness in a tangible way? Essentially, does the action cause harm to another person or their property without due cause? Does the action keep someone captive in some way without due process? These things should be what causes laws to be made. Now sometimes one person's liberties have to be weighed against another person's liberties to determine which one is most important. That is what we have courts for.