Sure, you've just finished watching one of the popular sci-fi shows and there was a scene where one of the regular cast had a pseudo sensual love scene with a quasi-human. His arms enwrapped about her body, their lips locked in an intimate moment of bliss and face to face they are having that almost love session. Better than 99% of all alien contact involving sex will be of a heterosexual nature; either species offering the male or female as the participants.
Have you noticed that most aliens are usually pretty humanoid? Ever wondered why? Easy. A sex scene between a human and some sort of hybrid aardvark octopus gets too difficult. Not necessarily awkward to write, but too complicated to convey in words.
It is our mind set. We can visualize a scene inside our brain but can't get the proper words to explain the intricacies of octopus suckers, tentacles and an aardvark nose and tongue melding with our human counterpart. (I'm sorry, maybe some may be able to imagine that.)
Of course, sex doesn't have to be necessarily with a space type alien. There are many other types that can be included in the mix: fairies, demons, robots, ghosts, Sasquatch, mermaids, and the list goes on.
Again, this list of possible love gods/goddesses will be nearly humanoid and have the corresponding sexual anatomy necessary for the love scene. This is where the art of writing and imagination can delve into the depths of wonder. This is where the alien doesn't need to be human in their sexuality.
What does this mean? Sex doesn't necessarily need to be the act as humans normally relate. Remember the movie "Cocoon" and the sex scene between Earthman Steve Guttenberg and Antarean Tahanee Welch in the pool? Steve experienced one heck of a sensation at Tahanee's sharing of herself with him. Another instance would be "Starman" with Jeff Bridges and Karen Allen. Again, somehow, even though it is not fully explained, Karen becomes pregnant and is carrying his child.
Back in 1983 there was a television series entitled "V" about visitors from space. It revolved around aliens coming to our planet. There were a few angles involving romance between these aliens and us. In fact, one story theme was about a young human girl and her alien boyfriend and their offspring.
Even if your non-human character is of the fanciful, demonic or mythological, the act itself can be described beyond the realms of mere mortals. Perhaps your mermaid and sailor can mate in the swirling waves of the ocean, your demon and innocent maiden make love in the throes of a fiery passion pit, or a leprechaun, by eating mushrooms, can become full sized to exercise his manly acts. Use your imagination to give your non-human creation the best possible sex.
Now exactly how sensual and sexual the science fiction and/or fantasy story gets is each writers own decision. Usually, in non-erotic type genres of sci-fi and fantasy, sex, the act itself, is normally downplayed with the strategy of foreplay getting most of the attention. I am not saying the story should be sanitized of sex but only if the act itself is important to the plot do you need to let your reader's libido run rampant. This is true of almost any element in the story; it must promote the plot or storyline.
Sex is the interlude and is best left to the imagination of the reader; build it up and then bask in the aftermath of the moment. Your reader can fill in the blanks if they know what it is about or left as blanks if the reader is too young to comprehend.
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