Where’s “Other”?

I just took a survey and the last question was asking me to indicate my gender, as shown below:


Now, I’m sure that there’s a perfectly good reason for them feeling the need to tell us survey takers to select only ONE answer for this, but I’m not sure I want to know what it is.  Did this particular survey designer have an issue with a lot of people trying to click on BOTH genders?  At least with that little bit of instruction I can narrow down my answer.  I have a 50-50 chance of getting it right!