Interbreeding

If you haven't read Larry Niven's essay "Man of Steel, Woman of Kleenex," check it out:
http://www.rawbw.com/~svw/superman.html

It analyzes the difficulties involved in a hypothetical mating between Superman and a terrestrial female, matters naturally glossed over in one of my favorite TV shows, LOIS AND CLARK. Niven deals at length with the problems posed by the probable bullet-like qualities of Superman's sperm, as well as his super-strength. Unless Kryptonians have self-control unknown to the men of Earth, he would crush Lois in his embrace "while simultaneously ripping her open from crotch to sternum, gutting her like a trout." Niven proposes an ingenious plan to get around these difficulties, using artificial insemination. He mentions but passes over more fundamental potential problems: Would Superman even find Lois sexually attractive, considering they belong to different species? What if Kryptonian women secrete sexual pheromones that human females lack? And wouldn't mating between them amount to bestiality (as Niven puts it, "sodomy")?

I maintain, in common with the guidelines of most erotic romance publishers, that if two individuals are sapient, they are "people" rather than "animals" to each other, so no bestiality is involved. As for the first question, Superman grew up in an Earth-human family. Like a duckling imprinting on a mother figure, he would identify with his foster-parents' species and would therefore find females of that species appealing. The really critical question about interplanetary interbreeding, however, involves genetic compatibility. What are the chances that an extraterrestrial would be fertile with a Terran? Niven suggests that Lois "could more easily breed with an ear of corn than with Kal-El."

Various authors present different solutions for this problem. It's not a factor in straight fantasy, of course (magic does the trick), but science fiction romance often has to deal with it. Some writers simply ignore it, as was common in older fiction. In Edgar Rice Burroughs' pulp classic A PRINCESS OF MARS, John Carter of Virginia manages to beget a son with Dejah Thoris of Barsoom, even though she lays eggs like a platypus. In my favorite H. P. Lovecraft story, "The Dunwich Horror," the hapless Lavinia is impregnated by the dark deity Yog-Sothoth, who is not only nonhumanoid but from an alien space-time continuum. Sometimes genetic engineering provides the mechanism for conception. It might be assumed that Sarek and Amanda had their half-Vulcan son Spock by this method, although Jean Lorrah's fiction (THE VULCAN ACADEMY MURDERS, THE IDIC EFFECT, and her "Night of the Twin Moons" fanfic series) postulates that the pregnancy occurred naturally. Another possible solution is a universe in which all human races throughout the galaxy were "seeded" by an ancient super-species and therefore have the same DNA. Susan Grant uses this device in her new novel YOUR PLANET OR MINE?, and the Star Trek canon contains hints of it. Although I don't say so explicitly, I assume in the background of my own vampire universe that organic compounds have been carried between planets and stars on comets and meteors, so DNA might be compatible among creatures evolved in different solar systems. In my current work in progress, I use the chimera concept (in the biological sense, not the mythological) -- the heroine's child has two fathers, one of them a monster from another dimension.

How would you handle the question of fertility between a hero and heroine from different planets or dimensions?