[logosmonkey]

I suppose there isn't really any way around it. It will probably also select for those who exercise more since they are more likely to have a stronger opinion on the subject. If you exercise a lot it likely has a strong net benefit for your life and you therefore value it to a greater extent than someone who doesn't exercise at all. By valuing it more you're more likely to engage in the subject. But that's probably true of all polls like this no matter the subject.