First of all, let it be clear that this is a setting specific question. I have no desire to discuss real-world morality and ethics. The setting has its own rules, its own gods, and its own laws of morality. Any argument about real-world morals (on all sides) simply do not apply here.
Now, how do transsexuals make sense in Pathfinder? There is no moral issue in play here, but I would think that a significant logical issue comes in; bodies can be magically modified to accommodate whatever gender you choose (or both or neither, for that matter). Further, it can be altered to be fully whatever gender you wish it to be, not just the outer parts. If you don't want your body changed, you could also theoretically have your mind changed.
So is there a reason why an adventurer would not choose to have physical gender match up with chosen gender?