In some (although not all) traditions of Wicca and Paganism, sacred sex is part of spiritual practice. Wicca in its original form is a fertility religion, first and foremost, so it's understandable that at some point you may encounter some references to sexual acts, whether they be actual or implied. By implied, we mean the symbolic -- the joining of an athame with a chalice, for example. The most commonly referenced form of ritual sex is the Great Rite, which is the ritualized connection of the god and goddess. Author Vivianne Crowley says, "The outer rite involves a linking of the male and the female… the sacred marriage is outwardly a marriage of two people, but inwardly it is a marriage of the two within one person." The Great Rite is more than just sexual union; it is the enactment of the creation of the universe itself in Wiccan tradition.
Although the Great Rite is certainly the best known form of ritual sex, not all ritual sex is the Great Rite. Ritual sex has a number of different purposes besides the Great Rite -- it can be used to raise energy, create magical power, or find a sense of spiritual communion with a partner. If "all acts of love and pleasure are my rituals," then certainly sex in ritual can be seen as a sacramental act of love.
It should also be noted that in some magical traditions, masturbation and sexual release is a perfectly valid way of raising magical energy.
In Her Hidden Children, author Chas Clifton writes, "Many Pagan religions include invocation and possession by their gods. Wicca, in particular, extends that possession to sexual acts, whether literal or metaphorical." He goes on to say that by making sex sacred, "Wiccans in all countries set their own stamp on nature religion, uniting in their bodies the cosmic and the most personal energies of planetary life."
Because ritual sex is a sacred act, any form of it should be consensual. In most traditions, it is also performed in private, and in all traditions, performed only by adults. Some traditions of Wicca require actual intercourse as part of a Third Degree elevation, or in rituals performed by a High Priest and High Priestess. However, most Pagans today would say thatit is never required by any legitimate coven for initiation as a neophyte. In other traditions, the act is symbolic but not actualized.
More often than not, if ritual sex is performed, it is between two individuals who are part of an existing relationship already, and who are of equal levels of power within the dynamic of the coven. Ritual sex between two Third Degree people has a good polarity to it… but ritual sex between a Third Degree and a Neophyte is stretching that balance of power a bit. Think of it as the difference between two teachers who date one another, and a teacher who dates his students.
Again, no reputable coven will demand sexual initiation of its members. Ritual sex -- the Great Rite or otherwise -- is a specific, sacred act that is performed only by those who have studied and learned enough to feel comfortable performing it with a trusted partner.