The nudity has nothing to do with sexual behavior. Yet some traditions have The Great Rite, a sacred enactment of sexual union usually between man and woman. This is true. There are covens that perform the Great Rite physically, and they do not break down into an orgy as part of their observance. Instead, one couple is usually chosen and left alone for a period of time.
Sex is sacred and private, sexual partners are chosen carefully with hopeful permanence in mind.
A seemingly unusual trait of female Wiccans is the celebration of the menstrual flow. If the cyclical mythology of the religion is examined, such an attitude about bodily functions is really not a surprise. Wiccans celebrate the full moon and a few celebrate the phases in between. When I first began practicing Wicca, I had problems with all of the above, but I was never ever forced to take part in any of the traditions that made me uncomfortable.