That depends on whether you want to maintain a friendship with the person asking the question. If you don't care, answer "When they change the stupid laws around here so I can marry my sister." That should shut up anybody within earshot. If it a friend you might just try, "Are you suggesting that a person isn't complete if he is single?"
Substitute genders as appropriate.
Shalom.
ROTFLMAO!
Could always move to West Virgina.