Sex and the Single Alien

The sexy alien has long been a media staple, but it has never been an accepted trope in literary science fiction which has higher standards of accuracy.  

If an author tosses in a sexy alien who is not only sexually compatible but also offers offspring, she might as well give up any hope of having people who read "real" science fiction as readers.

Aliens, according to many scientists, probably won't look like humans. You can create a reason why aliens do look like humans if you want to. I did for my race, the N'Videri in THE ONCE AND FUTURE QUEEN. (Note: ONCE is sf adventure/space opera so readers give you a bit more leeway than in standard sf, and I was counting on that leeway.)

In my worldbuilding, the N'Videri protected and nurtured those races who looked like them and pushed evolution for bipeds on certain planets so that they would have living camouflage for their own secretive presence in space. Humans, unlike many space races, were protected from conquest or self-destruction so they went safely into space.

It is extremely unlikely that a human and alien could produce a child. Human DNA has more in common with Earth grass DNA than it would have with any alien. 

Alien differences would also make breeding unattractive. For example, find two old pennies that are made of real copper. Wash them really well then put them in your mouth. The seriously nasty taste is what kissing Spock with his copper-based blood would taste like. 

Anything more than a small amount of copper is also toxic to humans and an effective, if hazardous, spermicide so there’s no way in heck that Spock would have been born even with scientific help.

Aliens that are essentially humanoid forms of Earth animals have their own problems. Fish reproduce outside of the body. (The female lays the eggs, and the male sprays his sperm on them then.) That's not exactly wild or possible sex for a human female. Lizards and snakes have a much cooler body temperature than we do. Part of the sexual excitement of the act involves heat.  A cold-blooded male in a hot-blooded female would be uncomfortable for both parties.  A hot-blooded male in a cold-blooded female might very well lose his erection because of the chill.

Aliens based on mammals have the danger of bestiality to be considered. Most of us are repelled rather than excited by reading about a human sexual encounter with a dog, big cat, horse, or whatever. Werewolves and other were-animals also have this problem unless they change into human form for sex.

I'm sure some people get their sexual buzz from all the above, especially in erotica which has little to do with logic or scientific accuracy and probably shouldn't, but any writer who wants their books read outside of a narrow category of readers has to consider such things.