There was also a story in which Lois and Clark pretended to be married in order to catch a criminal who was targeting newlyweds, but I can't recall the title at the moment. They were living in Clark's apartment with Clark sleeping on the sofa at first, but then they had to share the bed because they realized that there was a camera watching them. There was some anxiety over coming up with a cover for why there weren't actually having sex.
Another reason why someone might marry under duress is for inheritance purposes--some crazy old grandma leaves Lois a fortune, but she only inherits if she's married within, say 30 days or so. Trouble is, I can't see Lois or Clark actually doing that. I think Lois would rather lose the inheritance than marry for it.
Another possible reason for marrying is that Lois wants custody of a child (maybe Lucy's) and thinks she'd have a better chance with DCS if she were married. That would require killing off Lucy, or at least incapacitating her, and putting the child in danger of going to someone that both Lois and Clark consider dangerous (not difficult if you consider the crazy guys Lucy tends to date).
