I have read that both ser and estar can be used with casado to give different connotations to the phrase "to be married." What exactly are the differences, and when would you use each verb?
"Ser casado" can have two differnt "connotations"
- The passive form. You are married BY someone. Check this Groucho Marx quote in Spanish and in English.
I was married by a judge. I should have asked for a jury.
- It can also mean "to be married" in sentences like "soy un hombre casado" (I'm a married man) although in specific that case, I would say it's a pre-made sentence. No one would say (at least in Spain, which is where I am from) "soy casado".
We always use "estoy casado" ("married" being a condition that can change throughout the time). I believe that "soy un hombre casado" should be seen like "I am a man" (which I can't
changeeasily change) + an adjective.
Some other examples of this type of usage may include: "Soy un hombre íntegro" or "soy un hombre soltero". Forcing it a bit, you can say "soy un soltero" (although it may start sounding weird) but never "soy soltero". "Soy un soltero" is the same as "soy un hombre soltero" omitting the "hombre" part.
"Estar casado" means clearly "to be married".
This is purely my opinion (no "official" confirmation here).
Once in a while, the term "estar soltero" is used to describe a married man, usually with whimsical or humorous intent. It can range from something like the English slang phrase, "he's batching it this evening" to something carrying some innuendo.
This doesn't directly answer your question, but the use of "estar casado" when "ser casado" might have been expected can be sometimes heard as carrying a bit of humor with it.