Some time ago, we speculated here about what kinds of creatures (if any) would be completely unacceptable as romantic heroes. I recall a comment in a paranormal romance newsletter years ago (in the pre-Internet age) to the effect that scales or tentacles would disqualify a character as a love object. I protested: What about mermaids (scales)? What about Jacqueline Lichtenberg's Simes (tentacles)? On this blog we questioned whether anyone could fall in love with a slimy blob. Odo from DEEP SPACE NINE was offered as an example. On one of my e-mail lists, I recently mentioned a novel by Mercedes Lackey that includes a love affair between a human heroine and a sentient, vaguely humanoid (flightless) bird. I stated my dissatisfaction that we aren't given any details about his lovemaking technique. One of the people on the list expressed the opinion that a giant bird would be totally non-sexy.
So I'd be interested to read some new comments about whether there are any kinds of creatures that can't possibly be made sexy. How about a sentient spider? (Hmm. . . I haven't tried that one; maybe I will.) I made erotic use of tentacles in my Lovecraftian romance novella "Tentacles of Love" (Ellora's Cave). Conversely, what species make especially good alien romance heroes? Personally, I'm partial to shapeshifters (mainly werewolves or werecats), cat people, vampires (the intimate symbolism of blood-sharing), and Jacqueline's Simes (intimate sharing of life energy).
If you haven't read Octavia Butler's short story "Bloodchild," try to find a copy. It's not a romance, but it does focus on an intimate relationship between Earthlings and aliens. The insectile humanoids who dominate the planet have granted Terran refugees a colony on their world in exchange for one service: The giant-centipede-like females lay their eggs in the abdomens of human males. Typically, a female "adopts" a human family, for whom she shows genuine affection. And she tries very hard to remove her young before they grow large enough to eat the host from inside out. Butler manages the dazzling achievement of believably portraying a kind of love between the narrator, a teenage boy in a host family, and his insectile patroness. Butler was quoted as calling this a "pregnant man story."