directly from my research methods textbook

Wednesday, November 7, 2007 - Posted by Amanda Bast
"Another obvious closed-ended question is about gender. To leave such questions open-ended runs the risk that some respondent will either purposefully or inadvertently answer in a way that provides meaningless data. (Putting "sex" with a blank after it, for example, is an open invitation for some character to write "yes" rather than the information wanted.)"

Yeeeeeeah, I'd probably do that.