Reddit reviews X-Files: Trust No One (The X-Files (Prose))

The most common theory was that he was briefly married to Diana Fowley.

Wikipedia says:

In several shots, Mulder can be seen wearing a wedding band. This was David Duchovny's idea; he explained "That was just me, you know, fooling around. I had recently gotten married, and I wanted to wear it." He later described the situation as "so Mulder to never have mentioned that he was married". Series creator Chris Carter later told Duchovny that the situation "creates a problem. If we ever do a show that takes place seven years ago, you'll have to be married." However, Duchovny reassured Carter by pointing out that there were not very many episodes, if any, that had been planned to take place seven years prior to the events in this episode. The inclusion of the detail caused an "Internet frenzy" and the minor detail was never resolved on screen

And according to this book he was in a green card marriage and being the gentleman he is Mulder let her have the bedroom and that's how he ended up always on the couch.