The very serious Sa"nghaadisesa Rule (requiring formal meetings of the Community) of corrupting families1 concerns the proper relationship that bhikkhus should develop with lay followers.
It originated when two of the oft transgressing group of six monks neglected their Dhamma practice and behaved improperly2 in order to become popular with lay people. The lay people came to enjoy the sociable, playful monks so much that when more composed, right practicing monks came by they were considered snobbish and dull.
"If a bhikkhu corrupts families — in other words he flatters and fawns on lay people — and other bhikkhus drive him away from the monastery, and in return he criticizes them and if another bhikkhu then tells him that he must not do this, but he will not listen, a Sangha should recite the [formal admonishment] to induce him to abandon this mode of behavior. If he does not abandon it, it [entails initial and subsequent meetings of the Community]"
(Sa"ngh. 13; Nv p.7)
A bhikkhu guilty of habitually indulging in these practices (sometimes called vile and low conduct or paapasamaacaara) should be banished from his particular Community until he reforms.
Of course, a bhikkhu may concern himself in lay peoples affairs if it relates to religious duties. Also:
"the business of ones mother and father, or of persons who prepare to be ordained, called pa.n.dupalaasa (lit., yellow leaves, ready to fall off the tree), or of ones own veyyaavaccakara (steward, supporter, pupil) can be done by him even though these are apart from religious duties."
The relationship between the bhikkhu and his supporter should be a very special one:
"...A bhikkhu who is complete in good conduct does not lower himself to become the intimate of a family in the same way as a lay man may do. He is not aggressive or destructive but shows a heart of loving kindness and conducts himself in a moderate way, thus causing good faith and reverence to arise in them towards himself. He is then called kulapasaadako (one in whom families have faith). He is the splendor of the [Teaching]...
"Bhikkhus who are not strict lower themselves to become vile men but bhikkhus who are over strict are not interested in showing [compassion] in helping householders in various ways."
A bhikkhus wrong mode of livelihood also includes:
"running messages and errands for kings, ministers of state, householders, etc. A modern example would be participating in political campaigns."
"The term corrupter of families is one way of speech used by a group of bhikkhus. It does not mean that a bhikkhu gets angry, takes revenge, injures or destroys anothers wealth. The meaning is that a bhikkhu flatters by behaving as a layman, or by serving lay people, or by hoping for gains, giving a little to get much. Doing this a bhikkhu is called a corrupter of families because he makes lay people decline in faith which is the cause for the possession of skillfulness. Although a bhikkhu behaving in that way may please some laymen, yet they will not respect him as a bhikkhu, only treating him as an inferior friend. The term of bad behavior, refers to behavior beyond the bounds of a [recluses] conduct, for instance, playfully associating with girls in a family, or playing games, naughtiness or joking, singing and dancing." (Paat. 1969 Ed., p.157)2.
"They indulged in many kinds of bad behavior such as cultivating flowering trees, making them into garlands and sending them to women and girls of respectable families; eating and socializing with women and girls of respectable families; eating after noon; drinking intoxicants; dancing, singing and playing musical instruments; playing various games; training in elephant, horse and cart knowledge; training in archery and swordsmanship; wrestling and fighting; applauding dancing girls; etc." (HS ch.17)