True, there's a big gap between 100% straight and 100% gay which is, of course, where the bi's, ace's, pan's, undecided, etc. all fall into. A better poll would be "Are you straight, gay, or something else/in between?" Something like that
Ace isn't really the same as the others there, it's kind of on the whole spectrum but as another thing entirely since you can be straight and ace, bi and ace, gay and ace, etc.
As for the poll I don't think anything's wrong with it in terms of being exclusive since it's not "are you straight or gay"?", it's "are you straight or not?"