Technically nothing. Two homosexual siblings getting it on aren't going to produce children among themselves, regardless of how hard they try. Now if this was brother and sister, I'd be on my usual Eugenics rant right now.
If you have no set moral standard then nothing's wrong with it. If you have one then there is. But there's only one way to have one.
There sure is, but how do you manage to shove that stick all the way up your ass? I figure that would hurt, but it doesn't seem to bother you.
If you are a man or a woman your standard will never be fixed. Feel free to admit it or keep attacking me. Your choice.
My choice is to continue treating you like a trivial piece of flotsam that thinks it's an arbiter of truth and morality, when it probably contains the least truth and morality of those it judges from on high. You cock.
