The very fact that an incest taboo exists at all indicates that it was a common thing in society. History is rife with stories of romantic relationships between family members.
I've also heard of the taboo and forbidden nature can also cause some sort of reaction for people too which doesn't necessarily mean you want it as a real desire either but can still be arousing at the time.
Yahoo: Film Limbo: 18 High-Profile Movies That Were Shot But Haven’t Seen The Light Of Day
Film Limbo: 18 High-Profile Movies That Were Shot But Haven’t Seen The Light Of Day