I would say there's probably a good chance that the people on this board don't represent the overall socio-demographic layout of the wider population. Considering those who can afford more then one wife might be more likely to support polygamy then this might make a case for why the poll is not a good indicator. A lot of unknowns here, but I'm just putting it out there as a possibility.
There's a good cross-section of young and older on this board, so not everyone finds himself in higher income brackets. What may be unrepresentative is that most on here are prolly college educated. If college educated men have a different take on polygamy, then here is one bias in our poll. I'm not sure this is the case, though.