I just accidentally got this achievement. Here's the run-down of how it happened. When the notification came up I was shocked because I wasn't even trying for it. My Sim chatted up a Sim at the nightclub. While chatting, he had the options to talk about weird atmospheric pressure, do secret handshake, etc, so I knew she was also an alien. He did some of those socials. They ended the night as not quite friends. The next day, she called and asked to hang out. When she came over, they became friends and had a budding romance started. The only romantic interactions I did were flirt and compliment appearance. He then asked her to move in. After she moved in, he asked her on a date (which they did at home). During the date, more flirting and complimenting appearance, then first kiss. He asked her to be his girlfriend. At that point, I just wanted them to be able to sleep in the same bed. The next day, he got a whim to marry her. More flirting, compliment appearance, and other low-level romantic socials. He then proposed,they eloped immediately, and the achievement popped up. I thought it was weird because they did the alien socials, and she showed up as a "known alien," but she didn't know he was an alien.
