Alien love interests are a tricky business, especially if the author wants to have sf readers enjoy her book instead of tossing it across the room. You also have to deal with those of us with logical minds.
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.
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 for five minutes. The seriously nasty taste is what kissing Spock with his copper-based blood would taste like.
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 dumps 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.
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, 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 have, but any writer who wants their books read outside of a narrow category of readers has to consider such things.
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