I just wanted to share some explanation that was given by a teacher from Hinduism, Mr Kathirasan, about Shiva Lingam. Here it is:
"We can understand the Linga from the Puranic (Purana literally means old stories) perspective and the Agamic (or Vedic) perspective. Linga being Shiva & the base being Shakthi. When we pray to the Shivalinga, we worship the cosmic union of Shiva (efficient cause) and Shakti (material cause) that creates the Universe. The Shivalinga is like a snapshot of the creative potential of Shivashakti.
From the agamic (or vedic) standpoint Shivalinga is rupa-arupa. Which means that the Linga is amorphic. So it represents the nature of God. Ishvara (God) is arupa (formless) and therefore the closet rupa (form) to indicate the arupa (formless) is the shivalinga which is rupa-arupa (amorphic).
Hope this helps. It is highly meaningful if Hindus can go beyond the abrahamic religions' perception of union as something sinful and abhorrent.