Personally I don't think giving felons their rights back is the right thing. Even if it is a non-violent charge. Sorry you make a mistake of sleeping with a 13 year old girl (who you thought was 18) and you should be able to own a gun and vote? Who is to say that you wont go back to the parents house and kill them for sending you to prison? Sorry no. I don't believe that.
Who's to say that a law saying I can't vote or own a gun is going to stop me from doing whatever I want? If I'm going to commit the double homicide of your scenario then why would I have any problem buying a gun off Louie over here and doing it? Please, to even suggest that a law would make a difference is naive.