Superheroes have unique, demanding career paths that are un-relatable to many ordinary people. I witnessed similar relationships while in the military. Many relationships in the military struggle because the service member is constantly called on deployment, other duties in garrison, or mentally suffering from the innate hazards of the profession.
Look at Batman, the World's Greatest Detective. The original writers must have thought that there is not logical way for someone so focused on his training and quests to have a functional love life. So what would a single person of means do as they progress in life and age? Adopt a sidekick. And almost comically similar to real life single adults who adopt, there was much speculation into the Batman's sexuality by readers and critics. More recent Batman comics have given him an estranged biological son, but I'm not aware of any imaging of the character with a traditional family.
Not to fear, there are exceptions. One of the most successful relationships in comics is the interracial couple of Jessica Jones and Luke Cage. To two characters both work together and embark on their own missions in both film and on page. Jessica Jones and Luke Cage are partners both pursuing the same career so there's a mutual understanding of the duties and requirements of the job. How important is this to real relationships? I honestly don't know, but I'm wiling to do more research on it. Conversely, plenty of people have spouses in the same profession, only to find that relationship ending like any other.
The Marvel Cinematic Universe also glimpsed over another successful relationship in Avengers: Age of Ultron. It is revealed in that film that the character Hawkeye has a normal wife and kids living out in the country, almost wholly removed from his life as an Avenger. I'm no marriage counselor, so I'm only going to speculate why these examples work and how they might correspond to the real world. There's likely a large degree of trust, understanding, and respect that allows this relationship to be successful.
There is also the possibility that the issue behind the lacking love lives is a particular mindset founded upon personal ego. Superheroes may find themselves so devoted to their cause that they are unwilling to place priority on the feelings and needs of a particular individual. This would leave any partner to feel unwanted, unfulfilled, and unloved. We working professionals have dedicated so much of our time towards developing skills and pursuing promotions as if "leveling up" in a game. This might make us superheroes in our own minds, which may be a cool thought at first. However, I doubt we've thought much about the consequences on how that would leave us in the end.