The Śabdamātrā etc. (the cosmic sound principle i.e. all objects of enjoyment) evolved out of Prakṛti, being enjoyed by the sense organs; the word Bhoga comes to mean that which gives Bhaga. The principal Bhaga is of course the Prakṛti and Bhagavān is Lord Śiva Himself.
And hereafter the devotee shall worship the phallic emblem which is lord Śiva Himself. He is the sun giving birth and sustenance to the worlds. His symbol is justified in the coming into existence of things.