“jewishfeminist02: If they don’t file a joint return, they are required to file as a married couple filing seperately (which is usually expensive). If they file as unmarried (separate returns), they are violating the tax code and could be prosecuted if in doing so they pay lower taxes (as would be likely if they both have substantial incomes).”
As I said, it really depends on your financial situation. Sometimes filing jointly saves money; sometimes filing separately (as a couple, NOT as unmarried as I also already specified) saves money.
MarketWatch has a really thorough article on the subject if you’re interested. I can’t post the link, but you can Google “Your marriage and the IRS”.