In science fiction, and colloquially, xenocopulation usually refers to sexual relations with beings from other worlds, i.e., with aliens/extraterrestrials.
Several science fictions are rife with instances of xenocopulation, and in some cases, particularly Star Trek, have examples of hybrid offspring resulting from xenocopulation.
Scientist: "This is sure to be the beginning of a new era! Imagine the knowledge we'll share!"
Researcher: "Sharing the stories of times long since past!"
Engineer: "Those things are nice and all, but will the xenocopulation be any good?"
Spock: "As captain Kirk is preoccupied with xenocopulating with yet another alien female, I have the bridge in his absence."