What explains the "unsure" or "don't know" responses in polls?

When viewing poll results, I’m often surprised at the number of people who answer “unsure” or “don’t know”. Now, I’m all for withholding judgement until you have considered all of the evidence, etc., but there are some questions that seem rather binary yet result in a fair number of “unsure” responses.

For example, this poll at the top of the page by the Pew Research Center asked the following question:

“Do you strongly favor, favor, oppose, or strongly oppose allowing gay and lesbian couples to marry legally?”

The choices and results are as follows…
strongly favor/favor: 47%
oppose/strongly oppose: 43%
unsure: 11%

Let’s ignore the obvious question about the ethical deficiency of the 43% for a moment. What exactly could explain the 11%? I mean, what are the complicated parts of this question that would leave you unsure? Are you still evaluating evidence or something?

I don’t get it. As I browse through PollingReport.com, which gathers polls from a ton of sources, this response is common on many issues.

Is this just a result of people not wanting to reveal their response to the pollster?