This essay proposes that “milk kinship,” which upper-class individuals in premodern Japan formed with their milk kin—a menoto (wet nurse) and a menotogo (foster sibling)—occupies the core of an institutionalized erotic fosterage. In this “menoto system,” the surrogate mother's lactating body and erotic-affective labor became the connective tissue to bind two interclass families, creating a symbiosis that fortified the existing sociopolitical power structures. Around the tenth century, many vernacular tales started to feature menoto characters. While a typical menoto is the protagonist's homely, asexual, motherly confidante, her derivative construct—the menotogo of the protagonist—is often cast in an erotic light. In the four texts examined in this essay, menotogo valorize their erotic agencies to benefit their charges through sexual-affective labor or through an indirect method. The latter entails the formation of a “love square” in which two menotogo become lovers and then help their respective charges do the same.