IMPEDIMENT: Scenario

It's a wedding scene in a soap opera. The minister is saying, "And if there is anyone present who knows why these two should not be joined together, let him speak now or forever hold his peace."

In the back of the chapel, a stranger stands and announces to all, "She can't marry that man. You see, she's adopted. That man is really her brother!"

[Alternate Scenario: In the back of the chapel, a man stands and announces to all, "She can't marry that man. She's already married to me!"]

(crowd reacts in horror; organ plays dramatic crescendo; cut to commercial)

IMPEDIMENT: Explanation

In most cultures, it is taboo to marry a close relative because of the genetic possibility that recessive genes, e.g. for retardation, might emerge from inbreeding. In monogamous cultures, you only get one spouse at a time, legally. So either of these conditions--close relation or current marriage--would prevent a wedding.