Incest

Incest is quite a good topic for Being Bad, because we're all agreed that it's wrong, but no-one is quite able to say why. Even the person who anonymously confessed to a relationship with his or her sister recognises the impropriety of the act, or they wouldn't have cited it as something that they shouldn't have done. Some people, though, insist that incest isn't wrong. See here for several interesting articles about incest campaigning groups and cases studies
Forbidden Love by Johann Hari
Tainted Love by Ruth Elkins

The general confusion over just how wrong incest is is reflected in the variations in international law. Many legal systems have some kind of incest prohibition. But the degree of consanguinity which constitutes incest varies considerably. Some states even criminalise some forms of incestuous sexual relationship, while allowing marriage between such partners... Wikipedia surveys the laws of several states, while an analysis of international variations can be found here - http://www.mpicc.de/ww/en/pub/forschung/forschungsarbeit/gemeinsame_projekte/inzest/inzeststrafbarkeit.htm

Basically, it's a morality minefield...

The image is of Lot and his daughters. It's taken from here, and you can find out more about this Biblical incest story here.