I mean, think about it. You're facing yourself, and while doing so, you think that the other you is thinking the same thing you are. While you undress your opposite gender clone with your eyes, she too is doing the same to you. The exact same thoughts course through her as they cross through you.
Kissing her is like kissing yourself, as her lips even taste the same as yours. While penetrating her, you can almost feel the pain you are inflicting upong your own virgin self.
While doing each other, she knows exactly what pleases you and what doesn't, and so do you. She is inside your head, knowing every single last detail of you, as well as you.
I don't know about you, but I find the possibility distasteful.