my spouse has always been a U.S. resident but but turbotax requires me to say if i want to treat a *nonresident alien or dual-status alien* as u.s. resident?

This happens when a checkbox earlier in the My Info section is checked, even if inadvertently (and then unchecked).  The most foolproof way to resolve it is to delete your spouse and then re-enter under My Info.  

Be sure to make note of the spouse's Social Security number and birthday before deleting, if you don't have this information memorized.