The practice of ingestion of sexual fluids is mentioned by numerous tantric commentators, sometimes euphemistically referring to the penis as the "vajra" and the vagina as the "lotus". The "Cakrasamvara Tantra" commentator Kambala, writing about this practice, states:The seats are well-known on earth to be spots within the lotus mandala; by abiding within it there is great bliss, the royal nature of nondual joy. Therefore the lotus seat is supreme: filled with a mixture of semen and uterine blood, one should especially kiss it, and lolling with the tongue take it up. Unite the vajra and lotus, with the rapture of drinking liquor.According to David Gray, these sexual practices probably originated in a non-monastic context, but were later adopted by monastic establishments (such as Nalanda and Vikramashila). He notes that the anxiety of figures like Atisa towards these practices, and the stories of Virūpa and Maitripa being expelled from their monasteries for performing them, shows that supposedly celibate monastics were undertaking these sexual rites.