I know of lots of people who shouldn't be allowed to own a credit card, let alone a checkbook. Take a look at how the government handles money. If anything, people under guardianship for mental illness should be allowed to vote. After all, the state calls the shots in determining how people with mental illness will be treated. It's not unheard of for legislators to vote for and against bills based solely on which party drafted the bill.
Citizens routinely vote for candidates based on the candidates' appearance.
Of course, only a little more than half of eligible voters in Maine even bother to vote. What Question 5 comes down to is whether the electorate is ignorant enough to continue blatant discrimination that serves no compelling interest. Joe Six-pack sits in his trashy trailer down the road, guzzles beer, doesn't watch the evening news and doesn't read the newspaper. He's not likely to vote. But he can if he wants to, even if he has no idea what he's doing when he completes the arrow on the ballot.