Prostitution should be illegal because it can lead to the exploitation of women - this is harmful.
Incest should not be illegal, because it hurts nobody.
See, this is pretty easy. Then again, I define "immoral" as "harmful to others". Two guys making out doesn't harm anybody. Scarring a child is pretty harmful and pretty damn evil. They aren't morally equivalent, and your reasoning is as fallacious as it comes.