O-TSUNAGARI

The image of the linga in the yoni, which is the most common image of the deity in Shiva temples, similarly teaches the lesson that the tension between Shiva and Parvati is ultimately resolved in interdependence. Parvati as a sexual entity succeeds in tempering both Shiva's excessive detachment from the world and his excessive sexual vigor. In the form of the yoni in particular, Parvati fulfills and completes Shiva's creative tendencies. As the great yogi who accumulates immense sexual potency, he is symbolized by the linga. This great potency is creatively released in sexual or marital contact with Parvati. The ubiquitous image of the linga in the yoni symbolizes the creative release in the ultimate erotic act of power stored through asceticism. The erotic act is thus enhanced, made more potent, fecund, and creative, by the stored up power of Shiva's asceticism.

 source : http://www.bhartiyakala.com/article/parvati