It used to be thought that superheroes had to be single in order to be interesting protagonists. After all, a single superhero is a mobile superhero; it's easier to suddenly drop everything to go fly into space and have adventures with the Space Princess of Neptune when you don't have a wife and kids waiting at home. And besides that, everybody knows that comic book readers are lonely, single guys anyway. So why bother them with something they can't relate to, like a committed relationship? Compare Shipping Bed Death, which may lead to this in more romance-focused works such as Lois and Clark.