It was Stephanie Harbeson’s 21st birthday. She’d just finished opening presents when her boyfriend, Gary Hudston, sat her down in front of his computer and told her to get ready for a surprise. The screen bore a single image: the cake icon from Portal 2. Under it was a button marked “Play.”

Five minutes later, once Harbeson had navigated through some of the game’s physics-based puzzles, the sentient computer and lovable Portal antagonist GLaDOS had a message for her.

“Stephanie Harbeson, as part of a psychological testing protocol, I am required to ask if you will do Gary Hudston the honor of b...