the phrase is i wouldn't touch her/him with a barge pole - and it is a pole that is used to push barges along...a bit like punting i guess...therefore its long!!
Clanad, as I recall someone described the vertically-challenged film director (and alleged child molester) Roman Polanski as 'the original five-foot Pole you wouldn't touch someone with.'
oh, also... barge poles can get all sorts of slime over the far end as that's part of the job - so the implication is that the person you wouldn't touch is even slimier than the bottom of a canal.