Okay, you've got your Humanoid Aliens. But no matter what you do, they don't seem alien-y enough. So you add some Bizarre Alien Biology. What's the easiest way to do this? Sex! Guaranteed to appeal to somebody, and if done right, it can look like something other than just being a pervert or possibly Fanservice. It should be added that no matter how deviant it seems to us, Good People Have Good [...] [tvtropes.org]