Yobai (literally, "night crawling") is the nearly lost ancient Japanese art of a man creeping into a girl's house and having sex with her while the rest of her family sleeps nearby, often in the very same room.
According to a variety of historical evidence, this practice was widespread in rural areas well into the 20th century, although to what extent the practice may or may not continue to this day is unclear.
But in an era in which most houses only had one room, perhaps divided by paper screens at best, how realistic was it for an intruder to creep into the house and have sex without being heard by other family members? The truth is that yobai was often conducted with the tacit approval, or at least understanding, of the girl's parents or other family members.
At best, yobai was a way to get around proscriptions on premarital sex or adultery. For example, a young couple that was already fond of each other might "test out the goods" before being forever locked into marriage. Or in the case of a married couple who had fallen out of love or were never much in love to begin with, yobai might allow both the man and the woman to experience the excitement of other partners in a society where divorce was not an option.
At worst, yobai was a form of rape in which the woman had little or no say. In some villages there are records of fertility festivals in which the men of the village would get drunk on sake and go from house to house in a group, playing rock-paper-scissors to see who would get to go into the houses where young women lived and have sex with them, with the understanding that the girls would not be able to refuse their advances. In other cases, the village headman decided that certain girls in the village would be made available for yobai to important visitors to the village, with the girls and their families unable to refuse. There are also accounts of young men traveling to another village to engage in yobai, on the understanding that girls would be unlikely to refuse or make a fuss out of shame or embarrassment. Indeed there was even some discussion of how a girl should feel "honored" that a young man would go through such "trouble" for her.
What all of these scenarios have in common is that through the practice of yobai, some flexibility was created within prevailing norms against premarital or extramarital sex. As long as the man was gone by morning and no pregnancy resulted, there was plausible deniability for everyone involved that the sex never happened. Yobai even formed the basis of a traditional form of marriage called tsumadoi-kon (妻問い婚, "wife-visiting marriage"). After obtaining the assent of the girl's father, the man would visit the girl several times to have sex. After the first or second time, either side could call off the plan, but on the third time, the family would burst in and "discover" the couple, after which point the act would be considered public and the couple would be considered married. In some villages where wives remained in their natal homes after marriage, a husband wishing to have sex with his own wife would have no other option but to continue the practice of yobai even after marriage.
Indeed, evidence indicates that the term yobai originally came from the verb yobau, meaning "to visit repeatedly," with the kanji characters of "night" and "crawl" presently used to write the term constituting a folk etymology.
Although it seems likely that the art of yobai is mostly extinct in contemporary Japan, the practice remains very much alive in the imaginary of Japanese erotic fiction and manga and in "yobai"-style adult clubs and brothels, in which male patrons are allowed to "sneak" into a room where a stripper or prostitute pretends to "sleep" and playact a yobai scenario.