It seems I have my wires crossed, talking about a different poll entirely, the one that only asked Christians, distinctly politicised the question and was overtly negative.
The ComRes poll you have linked to is still flawed, but not to the same degree. In brief my criticisms of that would be:
1. First page questions are scene setting, getting your audience to think about marriage in a certain way. Asking to agree with a certain position (as you yourself argued in the Scottish Independance thread) automatically biases the answer. So marriage is important, best fit for children is mother and father is already setting up a rather biased view of what marriage should be.
2.Second page question point out that gays already have civil partnerships. So they final question on that page is again asking to agree after already setting up that yes, marriage is good, yes kids are better with a mother and father and yes, gays already have recognition via civil partnership, so you agree that we should keep marriage as a man and a woman?
It seems to be a selection of questions designed to illicit a certain response.