Apparently, the hottest thing in the Multiverse, above even the Cute Monster Girl, is the Human Female. Sooner or later, no matter what the species looks like, you'll find a sample of the male that prefers human girls. It can be an alien, a swamp monster, a robot, a Non-Human Sidekick, or even a giant gorilla. (Sometimes it Was Once a Man, which makes the attraction less implausable.) Note that this is usually played straight, while the inverse is almost always played for laughs. Also, the male is far more likely to actually get the girl. This is one of the reasons Aliens Are Bastards.