I'm only getting into semantics because you are - that statement is singular and definite, therefore it can only be taken to have one of two meanings:

A) a given straight man is selected at the start, and once he is voted out it ends.

B) it ends once the final straight man is voted out, as at that point he would be the straight man. This can never occur as he would win by being the only one remaining, so there would be no vote for him to vote himself out.

If they're all straight without a single selection at the start, then there is not a definite straight man, and even if there is one selected at the start there's only a 1/12 chance he gets voted out first.

I mean, dude, obviously that's just the version of the rules that you're giving to the competitors.