Thanks. I read both of those books back in the 70’s, so my memory is a bit hazy, and I think it might be Titan, though I’m not positive.
The human male hero fell in love with his centaur traveling companion. He engaged in sex with her-it, mainly with the centaur’s upright front torso section. that didn’t freak me out too much. Eventually, though, she-it talked him into doing it with her hind quarters, as she-it drew pleasure from that area, as well, which is were I went “EEeeew!”.
Think I’d need a bit of alien booze first.
Yeah, I think that was Titan. The Titanides have two sets of genitals, front and back, IIRC. Which is why their reproduction’s so darned complicated…I think it’s mix & match, at that…a single Titanide can be male in front and female in back or vice versa, as well as having self-matching genitalia.
SciFi books never mention smell. Smell is probably the hugest sensory turn on/off for me.
Any alien that smells like cinnamon would get a come hither wink.
I’m imagining Jabba the Hutt spritzing with Eau de Sticky-bun and going looking for you…
Is that a Cinnabon clutched in your chitonous tentacle, or are you just happy to see me?
Most Star Wars aliens are out.
Most Star Trek aliens are in.
The alien from *Alien * is right out.
Rishathra?
Sure, why not?
Despite what goes on in Star Trek, most alien sex probably wouldn’t even be recognizable as such, or would be as different from ours as Plant sex, or jellyfish sex. As Larry Niven wrote in “Man of Steel; Woman of Kleenex” regarding Superman getting it on with LL, “He could as easily mate with an ear of corn.”
Larry Niven’s rishathra is understandable, since the aliens really are humanoid, and of the same basic stock. For others, though, it doesn’t seem likely. Although there have been attempts:
1.) Norman Spinrad wrote a short story (don’t recall the title – it’s in the collection The Last Hurrah of the Golden Horde) in which there’s a planet with a species that uses some sort of telepathy to find out what sort of mate a creature is looking for, and forms itself into that ideal. It’s sort of cuckoo-like survival strategy, where you displace the natural females. The same idea (sorta) was used in the movie Species and its sequels.
2.) Philip Jose Farmer came up with an interesting dodge in The Lovers. The alien female creature looks like a human female and can go through the actions of sex, but of course we’re not interfertile with them. Nevertheless, the sex act stimulates their reproduction mechanisms. It’s high;y unbelievable, but a good read. I don’t recall how these were supposed to habve developed.
3.) another Farmer story, “Mother”, features a huge immobile creature that has a large internal cavity where it can trap things. It traps a human. There’s a “reproduction spot” which, when attacked, is stimulated to begin reproduction. This has to be the most complete “return-to-the-womb” story ever written.
Anything stranger than M’Ress is…mostly…out.
Ok. Thanks for the confirmation. 
I am getting old, and my memory is not like it used to be.
They are both male and female in back, and can have either male or female genitals in front. Frontal sex produces an egg, which must be manually placed in the rear female organ and fertilized. They are both physically and genetically compatible with humans frontally; designed that way.
Thank you! It’s been a good 15 years since I read that trilogy and I thought I was getting it wrong somewhere…
Wait, are they genetically engineered, winged, flame-breathing ALIEN SEXY monkeys?
(Hmm… flame-breathing AND winged might mean very few acceptable positions… have to consider this)
Wait, animated M’ress or Live Action M’ress?
(Live Action Caitian: Caitian | Memory Alpha | Fandom )
The Caitian in the live action photo is not M’Ress, & is Male.
I don’ swing that way, dude. 
No, they’re genetically engineered winged flame-spewing venom-spitting poo-hurling howler monkeys with lasers on their heads. If you think that’s sexy, you need help.
Unfortunately, the photos on that site are blocked for me.
Did Memory Alpha dot org decide to go with photobucket when I wasn’t looking?
What if the male has barbed penis, or they spray piss on the curtains, scratch up the couch, lick their genitals in public, or any other weird cat behavior traits?
I could see getting turned on for Jadzia, Kera, the Orion slave girl, or any of Kirk’s alien girlfriend of the week. Hell, I’d even lick the clitoris on Ensign Ro’s forehead. But an anthropomorphic cat, a Klingon, a Gorn, Ferengi, Cardassian, gecko, skink, Chewbacca, Jaba the hutt, Greedo, E.T., a centaur, or a unicorn is out of the question.
There were some hot slags in Alien Nation, but again we’re talking about a human with a watermelon scalp. And I hear that gender doesn’t matter if it’s Arcturian…
Aaaah, but are they female genetically engineered winged flame-spewing venom-spitting poo-hurling howler monkeys with lasers on their heads? With big boobs? Because then, you know. 