Sci Fi characters you may want to sleep with but probably shouldn’t
Science fiction films draw a certain type of person. That person tends to be interested in adventures and space and the unknown. Also, depending on the intensity of a given science fiction fan they may be a virgin, overweight, and possibly live in their parents basement.

If Bill Gates and Mankind had a kid
A trademark of any science fiction movie is some sort of extra terrestrial life. Some of them are human looking, some are hot, and others are just completely ridiculous. Here are a few aliens from different movies that you may or may not want to hook up with and why it’s probably a terrible idea.
Mystique:
She was the shape shifting mutant from the X-Men. She was hot in the cartoons and was made extra hot in the movies when she was played by Rebecca Romijn Stamos. I think she was able to mimic some powers such as Wolverine’s claws so I would assume that ability can transfer to looking like someone with enormous breasts.
Except she's blue
She was also on the ‘bad guys’ side and fighting against the X-Men but I’m sure if you were both big Magneto supporters things might work out between the two of you. Then again she has killed quite a few people and will do whatever Magneto says so make sure you don’t cross him. Also you would have to stay on your toes because she could come onto you as some other hot chick. And you could cheat on her. With her. That’s no good for anyone.
Sil:
This is the alien from the movie Species played by smoking hot Natasha Henstridge. For most of the movie she looks like a super model and is only about 5 months old.

Is this considered child porn?
Not sure if that means she will be old and decrepit by the time she is two years old. Let’s assume her species has the ability to look like a 24 year old Swedish model for as long as they want. There is really no upside to bedding this gal. Best case scenario you’re a virgin with a terminal disease and she decides to kill you after sex. Unfortunately if that’s the case she probably won’t want to use you to procreate. Even if she does I’m sure there is some awful process a la Alien when making alien babies. Oh I almost forgot to mention she actually looks like this…

No one wants to be part of making any more of those. If you see her coming try to hide cause you won’t out run her. If she does catch you just be happy you won’t be around for the inevitable apocalypse once her kids hatch.
Jenny aka G-Girl:
This is Uma Thurman’s character from the film My Super Ex-Girlfriend who like most her roles can look very hot while she is trying to kill someone. She is a super hero who gained her powers from some random meteor she got too close to when she was a teenager. In case you thought this movie sounded really bad already then you would be correct.

She almost looks good enough to make me see this movie
Her non super hero persona is very shy and not very good around guys. She meets Luke Wilson whose character has mastered the art of making even the most insecure women comfortable. She gets super jealous when she sees him with another girl and shenanigans ensue. At one point she throws a great white shark at the couple. Toward the end she loses her powers, then gets them back, but so does the other girl and then they fight but make up, and Jenny ends up dating her arch rival from throughout the movie. Or something. Look avoid this whole situation. She seems to be a drama queen and totally not worth the hassle.
Serleena:
This is the antagonist from Men in Black II. Let’s start off with the pros. She is a shape shifter who is posing as a lingerie model.

That means if you wanted to sleep with a female version of the kid from the Whopper Jr. commercials, she can pull that off. She’s also a go getter and an independent girl so that may work for the stay at home dad fellas out there. Unfortunately she is a bit sadistic and really has no problem hurting and killing people. So once again as long as you’re sure you won’t piss her off it might be worth a shot. I mentioned she is a shape shifter which was a pretty attribute for her species since their true form is that of a Lernaean Hydra aka the one Kevin Sorbo fights in Hercules.

Great show
In the movie she manages to somehow take that form while wearing her pants suit which is a little difficult to explain. My recommendation would be to stay away from this one.
Cylons:
Now for anyone who is a fan of the original or new Battlestar Galactica series I am talking about the hot ones from the new series and not the Tin Man 2.0 version from the original series.
If I only had a brain
These Cylons were meant to look exactly like humans in every way which was so much easier than buying costumes for everyone. The female ones were also drop dead gorgeous and completely capable of having sex. The downside was that they can’t always produce children. So you’re really rolling the dice if you’re trying to start a family and you would probably have to move to a state where that kind of thing is legal. On the up side you never need to worry about a rubber. The other risk you’re taking is that these are machines and much stronger than you. So if you find one with a short temper you may end up getting hurt and if you’re into spousal abuse you have an unpleasant surprise coming your way.

This one is definitely a toss up
So as you can see looks aren’t everything. You have to consider tentacles, aggressiveness, and any desires to eradicate the human race. If any aliens have the technology to get here they most likely have the technology to destroy us. Some of them decide to give us some eye candy before they do it which is considerate. As a rule of thumb if you’re the type of guy who can’t get semi attractive girls to notice you and one that looks like a super model seems very interested, then my advice is to assume she wants something from you and it won’t be pleasant.