Quinnipiac is biased toward the left. Also, note that the respondents say that it’s OK for gays to serve, but that they should “limit displaying their sexual orientation on the job.”
In other words, they don’t quite understand the question, since that’s pretty much what “don’t ask, don’t tell” is. Clinton’s policy was actually a slight liberalization, since the previous policy was theoretically that it was wrong for gays to be in the military at all.
Also, as already said, I doubt whether they phoned any military bases to ask for responses. So they may have gotten a few vets, but the poll still was biased—in addition to their usual leftish bias.