In a miscellaneous talk, Dogen said,
Men and women in secular society, both young and old, often pass the time talking about lewd things. They do so to amuse their minds and beguile themselves. It seems as though idle talk entertains their minds and diverts them from boredom for a while. Monks, however, should completely avoid such talk.
Even in secular society, when well educated and sincere people discuss some serious matter with due courtesy they do not engage in such talk. They do so only when they are drunk or unrestrained. Needless to say, monks must concern themselves only with the Buddha-Way. Only a few eccentric and immoral monks engage in such indecent talk. In the monasteries in China, since they never engage in small talk, they do not speak of such matters. In our country also, while Eisai, the Abbot of Kenninji Monastery was alive, one never heard such talk. Even after his death, while a few of his disciples were still at the monastery, people did not speak of such things. Lately, in the last seven or eight years, the young monks sometimes indulge themselves in idle talk. This is really shameful.
In the Scriptures, it is said, “Though coarse and violent actions may sometimes cause people to wake up, worthless speech obstructs the true Way.” Be it even a word which comes to the lips unintentionally, useless talk hinders the Way. Still more, lewd talk will excite the mind. You must be most careful. Without forcing yourself not to use such language, if you realize it is bad, you will be able to reform gradually.