So I realized that the sexy aliens trope is only plausible if either we just colonized a lot of other worlds, and are just boffing with humans who've been on other worlds for a while, or (more likely) captured aliens and made genetically engineered versions that fit better into human society (which included making them super-fuckable).

This is how you get aliens in which the females are shorter, slighter with more rounded features and boobs. Contrast the whole of the zoological catalog on earth.

But we humans want aliens we can have sex with.