Nan's
The Newlywed Game is a terrific follow-up on the pheromone-induced wedding idea.
CarolM has a red-K-induced sudden wedding in
Falling Slowly .
A second 'obscure Lexor law' story is
Lois and Clark and the IRS Regarding the movie scenario--it's not the ceremony that makes you legally married, IIRC--it's the license. It would be hard to accidentally sign a marriage license. Maybe it's different in Nevada.
Here's a scenario I've thought of for a reluctant marriage: Suppose that, at some point in their dating relationship, Lex won't take 'no' for an answer and Lois is the victim of a date rape. Not only does she have the trauma of the rape, but she ends up pregnant. Meanwhile, now that her Lex blinders are off, she's more willing to listen to Clark on the Lex is evil front. Clark, with his supersenses, realizes that Lois is pregnant pretty early on. Here's where the marriage comes in: In most states (and since New Troy is fictional, we can use whatever state law we want), when a married woman has a child, her husband is assumed to be the child's father. That could lead Lois and Clark to get married for the sole purpose of protecting her child from Luthor, who would, of course, want custody out of a sheer desire for control. Much potential for angst in that scenario, and a lot of difficult decisions for Lois, starting with why she chooses to keep the child in the first place. Many mixed feelings for Clark, too, who would be torn between loving the child because it is Lois's and trying not to resent it for being Lex's, with much ensuing Clark-introspective-guilt.
