A subtrope of Human Sacrifice, Virgin Sacrifices were generally made in ancient times as payment to a God or some other power, either to keep them functioning or to win their favor in general.
Tribes in Africa and South America engage in ritual Virgin Sacrifice every fall for a good spring harvest/hunting season.
A ritual performed at most live showings of The Rocky Horror Picture Show. People who have never been to a live show before are virgins. Most casts will choose several virgins out of the group and do all sorts of crazy things to them.
Ann had never been to a live showing of The Rocky Horror Picture show, so she was picked to be part of the virgin sacrifice.