In a culture with some rather strict notions of whom it is fit to marry - i.e. probably more strict than 'don't marry close relatives', imagine if the language has a register whereby if you speak to an unmarried, young person whose family relations you are expected to be knowledgeable about, you are forced to use a register whereby anyone forbidden to this young person is in a special noun class.
(I also imagine the culture could have other, non-genetical things that determine permissibleness, for instance, astrological or other similar superstitions.)
After the young person has married, this register is no longer used with him or her. However, the congruence markers can still be used for a few specific meanings: acts of fornication and general violations of the sexual mores of the culture will always have that noun class's congruence marker on the verb.
Here, the marker also is unique in being the only such marker to permit reduplication; when doing so, it is intensive and judgmental, essentially conveying something like 'you fucking idiot really had to fucking go and fuck your cousin did you?'
Finally, in some minority communities with other taboos as well - including more formalized food taboos - the same morpheme has come to mark violation of any of the other taboos, regardless of noun class of the object, or even with objectless actions that are forbidden.