Heard about this in a BBC documentary (podcast).
Mount Kemukus or Gunung Kemukus (known as Sex Mountain) is a hilltop Javanese shrine in Sragen Regency where people from across Indonesia flock to honor a local saint. Every 35 days, rituals are conducted that involve adulterous sex.
The ritual involves first, prayers and offerings of flowers at the grave of Pangeran Samodro and Nyai Ontrowulan. Pilgrims must then wash themselves at one of the sacred springs on the hill and find a stranger and have sex with them. They need to have sex seven times. That is every 35 days, so it is a relationship that lasts around a year. So the couples have to make a significant commitment to each other. They need to exchange mobile phone numbers and addresses and decide where to meet again, so they can complete the ritual. Most people who do the ritual are small business owners. They hope that if they carry out the ritual their business will flourish and they will be successful.
https://www.youtube.com/watch?v=Hxx_NGOalfI
The shrine is that of legendary prince Pangeran Samodro, son of a Javanese king, and his stepmother Nyai Ontrowulan. Legend has it that they ran away together and lived at Gunung Kemukus. It is believed that if you do something even more shameful there, like have adulterous sex, then you will be blessed with good fortune. It must be done on the auspicious day of Jumat Pon, when the Friday on the Gregorian calendar intersects with one of the five days of the ancient Javanese calendar.
According to Floribertus Rahardi, an Indonesian writer who has studied the ritual, “People believe that they [Pangeran Samodro and Nyai Ontrowulan] committed incest in that place, but before they had finished having sex they were chased, killed, and buried together in the one hole. From there, word emerged that whoever can finish off their sex act will receive blessings from Nyai Ontrowulan.”
The ritual is not seen anywhere else in Indonesia or the rest of the Muslim world. It is a very Javanese blend of religious ideals with Islamic, Hindu and Buddhist influences.