[3niner]

Adultery and incest are two of the most destructive behaviors to a successful civilization.

Is adultery illegal? There are other destructive sexual behaviors which are not illegal, also. This is why I'm surprised.

[sodpoodle]

Adultery used to be considered bigamy; ergo illegal. i.e. cohabiting with someone other than your spouse.

http://legal-dictionary.thefreedictionary.com/marriage