It expands to "No Biomechanical Barriers". The same arc suggests Bunnie, who is robotic from the waist down, will have children, too!
Actually in certain continuities, Belle and Jacques (Bunnie's kids) could actually be born normally (so to speak) as Bunnie is cured of her robotization...though given that plotline, some fans will likely ignore it.
It is averted in the case of the Iron King and Iron Queen, a massive ox-man and ordinary human woman respectively, whose "marriage" is purely an arrangement of political convenience; the writer acknowledged that having a human and animal character wed "might raise some eyebrows".
There are other Titan couples as well, such as Superboy (human/alien hybrid) and Wonder Girl (demigod).
Professor Charles Xavier and the Shi'ar Empress Lilandra in X-Men.
Made worse by the fact that Lilandra is descended from avian stock, Charles from primate stock. OTOH, the Celestials might have reengineered both breeds to make them compatible. The Shi'ar seem more primate-like than evolution would plausibly manage...
Karolina (Majesdanian) and Xavin (Skrull) in Runaways. When Xavin was Put on a Bus, Karolina and Julie Power (human) started going out.
Nova (human) and Green-Skinned Space Babe (literally) Gamora, in Annihilation. Before (and after) Gamora, Rider was with Namorita (Atlantian Mutant/Clone). Namorita's metamorphosis into Kymera (much less human looking form, resulting from her ancient Atlantean DNA) didn't ruin this relationship.
For a long time, the X-Men had Dazzler (human mutant) and her husband, Longshot (interdimensional alien). Since their divorce, Longshot seems to have jumped on the Boldly Coming bandwagon of interdimensional relations.
Depending on which origin story you actually believe, Rictor (human) and Shatterstar (alien-ish.) are this.
The former might not be an example, since the Vision might have been designed to be compatible. Is an artificial person of human design a different species? As for Crystal and Pietro, the Inhumans and mutants are two branches of one breed anyway, so that doesn't really fit the trope.
Later, Crystal was remarried to Ronan the Accuser (Kree).
Namor (human/Atlantean hybrid) married Green-Skinned Space Babe Marrina, but also found time to pursue the human (and all-too-married) Invisible Woman.
Namor and that lake-monster queen from Tabula Rasa
Hope: Ick! Ick! Ick!
Namor: You are only showing your parochialism and prejudice. She is a dignified queen of a noble people. Namor is nothing if not cosmopolitan in his tastes.
Also, there's been flirting and suggestions of future hookups between Namor and Hepzibah (an alien skunk woman. Well originally a skunk woman, a lot of artists these days tend to draw her as a Cat Girl or even just a palette-swapped human, but whatever).
Atlanteans in the MU are a branch of the human race, so this would me more like interSUBspecies romance.
Namor, as implied above, is the result of one of these pairings: human father and Atlantean mother. His cousin Namora has a human mother and Atlantean father. As they're effectively the only members of their kind, most any romance they pursue (aside from each other) is one of these.
One of the most exteme cases in the Marvel Universe was Thena (one of the Eternals) and Kro (one of the Deviants), a true Monatgue and Capulet romance, seeing as their two races were sworn enemies.
Smallville addressed the essence of Niven's concerns by noting that the "training" that Jor-El had arranged for his son included "control" in that specific aspect. Apparently Clark's parents were expecting him to get involved in an Interspecies Romance and prepared him for handling the tricky aspect of intimacy.
Also from DC, John Stewart married Katma Tui (alien) and had a long-standing relationship with Merayn Dethalis (also alien). Justice League paired him up with Hawkgirl and featured Katma as an old flame who he was still on good terms with.
The Green Lantern Corps seems rife with this. Hal Jordan and Arisia, Kyle Rayner and Soranik Natu, and when you bear in mind that nearly every member of the Corps is a different species, when relationships between members of the Corps were banned over 200 resigned in protest.
Mossy earth elemental Swamp Thing and his human wife Abigail Arcane. Played with considering the nearby town finds out and convicts her of bestiality. Abby flees to Gotham City where the police hold her, causing Swamp Thing to go ballistic to free her. Batman finally talks the Mayor into giving in considering that if he wants to hold Abby for having a relationship with a non-human sapient being, then he'd better arrest Lois Lane, among others, for her relationship with an admitted extraterrestrial, Superman. Since busting the world's greatest superhero's girlfriend for bestiality would look very, very bad and ridiculous in the media, the Mayor instantly gives in.
Among the first Supergirl's boyfriends were a human, a green-skinned Coluan, an Atlantean merboy, and, er, her horse. (He was in human form at the time, admittedly, but there was still a kind-of relationship between them after he was turned back; it probably helped that he retained most of his intelligence.)
Pretty much the point of the Barbarella comic strip and carried over, to a lesser extent, in The Movie.
ElfQuest: I'm gonna have to spoiler this whole thing because it involves a major plot revelation. In ancient times Timmain (super-advanced humanoid alien in elf form) shapeshifts into a wolf and mates with the alpha male of a wild pack. result: Timmorn Yellow-Eyes, half-elf, half-wolf and father of the Wolfrider tribe. Many generations later the pure-blooded elf Winnowill views the Wolfriders' origins with disdain, asking the pertinent question: "Do they renew their kinship even now?"
This disdain is hypocritical since Winnowill is soon revealed to be the mother of Two-Edge, who is half elf/half troll.
There are also some attempted human/elf pairings, which generally go badly because the elves' "bloodfire" is too intense for humans, although centuries after the main storyline the shapeshifting elf Jink enjoys a number of successful partnerships with humans of both sexes. Since she does seem to have a weakness primarily for telepaths, it may be that the elf/human compatibility issue involves a psychic element...that, or she just has worked out the kinks through experience.
Legion of Super-Heroes: there are a lot of interspecies romances in the Legion; some of the more notable are Brainiac 5(Coluan)/Supergirl (Kryptonian), Colossal Boy (Human)/Yera (Durlan).
Just about all major cast relationships are Cross-species; the Legion is compromised of tons of different species hanging out together 24/7, almost all raised with no species boundaries whatsoever.
The Archie-published Teenage Mutant Ninja Turtles Adventures featured three such relationships. The first two involved mutant turtle Raphael, who became involved with two mutant fox women at different points of his life, and even ended up marrying one of them. On the other hand, there was the relationship between musician Mondo and his girlfriend Candy Fine, which persisted even after Mondo was turned into a mutant gecko.
Not to be left behind, the original incarnation of the TMNT has featured two different interspecies romances. The first occurred between Michelangelo and "Seri" (reptilian alien), while the second will occur between Leonardo and human superheroine Radical several years into the series' future.
However in Teenage Mutant Ninja Turtles (2012) Donatello (Donnie) is in love with April O'Neil April O'Neil is soon revealed to be actually a MUTANT of half Kraang Alien/half human. Also there is Leonardo(Mutant Ninja Turtle)& Karai (full human) However since Karai is part of the Foot clan, they had become rivals especially since Karai is actually Miwa & the daughter of Master Splinter & it is soon revealed to be the daughter of Master Splinter & his wife Teng Shen. Though she does not know it herself, though Leonardo was clearly very attracted to her.
The downsides to be explored a bit more in others. For instance, one story features a Gotal who enjoys sleeping with members of other species. Or he did, until he slept with a female from a species that eats the male alive after sex.
The Rogue Squadron series had a romance between a Calamari and a Quarren: Romeo & JulietIN SPACE!, in short. And in one arc it was implied that Janson and Hobbie slept with a pair of Bothans.
One of the most prominent romances of the Clone Wars comics was between Jedi Masters Tholme (human) and T'ra Saa (Neti). Neti are extremely long lived, shapeshiftingplants.
At least one of the comics set during the prequels had a fair bit of Ship Tease between the aquatic Kit Fisto and Twi'Lek hottie Ayla Secura. Talk about Naughty Tentacles...
It's pretty much taken for granted in the Star Wars universe that Twi'lek women are desired by males of all species - and yes, it often really is due to their "naughty" lekku, which can be trained to perform sensual functions. It was used rather disturbingly in A. C. Crispin's novel The Hutt Gambit, where a young Han Solo is (to his chagrin) leered at by an elderly female Twi'lek.
Cade Skywalker (Human) and Deliah Blue (Zeltron) from Legacy.
2000 AD'sLobster Random ... a mechanophile. That is a (modified) human who is sexually attracted to robots. And not human looking robots either...
Astonishing X-Men: Beast/Brand. Complicated, since he's a human mutant who became more animalistic as he got older and now looks like a giant blue cat, and she looks mostly human and is actually half-alien. Apparently her dad was a huge blue furry thing, which takes it into even more perverted territory.
In the Donald Duck comic "The Old Castle's Secret," the jewel thief Diamond Dick claims that he knew about the hidden McDuck treasure because his "third wife was a McDuck on her great grandfather's side." Diamond Dick is a dog. Apparently something... interesting... has been going on in the McDuck family tree.
Toyed with in Frank Miller's The Dark Knight Strikes Again, where Hal Jordan whose control over his powers now renders him virtually omnipotent, has left Earth and is now living on Mars, where he lives a happy, wholesome family life with his wife and son. Unlike most examples of this trope, the Martians are not rendered physically attractive or appealing in any way; Hal Jordan instead uses his power to make himself a Martian, completely capable of maintaining the form indefinitely, as well as capable of breeding with other Martians, and is extremely reluctant to return to Earth after receiving Batman's transmission, until his wife urges him to help his own people. He only reverts back to his human form after he has already left Mars, and is quick to return as soon as his business on Earth has been finished which involved him multiplying his size to that of many times the Earth, able to grasp the entire planet in his fist, crushing Lex Luthor's planet wide multi-trillion dollar WMD satellite network while leaving the planet itself unharmed.
Oh, Tank Girl, as if your sociopathic heroics weren't enough, you also have to get involved with Booga. The mutant kangaroo. And then cheat on him with your tank... Yeah, it's that sort of comic.
ROM: Spaceknight was in love with the Terran Brandy. On the plus side, Rom is a Galadorian, basically a branch of the human race. On the minus side, he'd been cyborged into an all-covering suit of plandanium armor. On the plus side, it was theoretically possible for him to become a normal man again. Still, whether Rom and Brandy count as an interspecies liaison is a judgement call.
DC once printed an Elseworld story (back when they're still called "Imaginary Stories") where Lois Lane had been killed after Superman settled down and had a daughter with her. At one point, he created a robot duplicate of her, which proved to be all too lifelike◊. Apparently, Uncanny Valley doesn't faze some people.
Werewolf by Night's Jack Russell (a werewolf) has had several long term relationships with human women.
B.C. has John the tortoise and Dookie Bird, who also had a fling with a pelican at one point.
Rocket Raccoon (take a wild guesswhat he is) and quite a wide variety of alien species. In his original mini-series, his girlfriend was Lylla (anthropomorphic otter). In his 2014 series he has been shown to have a brief relationship with Princess Lynx (alien, cat-girl). In the 2014 Guardians of the Galaxy annual, Rocket is shown to have 3 sons (which he insists are nephews) whose mother is clearly some sort of blue skinned alien.