How do you know it was done without consent? Were you up in that spaceship with your Sim? Perhaps he willingly agreed.
Also how do you know any act of intimacy occured? Maybe it was a medical procedure done under anesthesia.
It could well happen like this: Your Sim is very interested in alien life. He has been studying them with the telescope and has even attempted to contact them. Finally they answer the call, and appeal to your Sim to aid them in their pollination program, which is now entirely consensual. All it requires is a short medical procedure and the willingness to give birth. At that point your Sim also decides whether to raise his alien baby on earth or to send it to a nursery on Sixam. Either way your Sim can bask in the knowledge that he helped keep an alien race from going extinct.
Admittedly, a little cold hard cash in the form of child support would be nice.
The point is any scenario in the Sims is only limited by your imagination. If, on the other hand, you want to dream up an evil planet of body snatchers, you can do that too. That's the beauty of it.
