Q: It cannot be denied that it is sexual desire that causes man to engage in coitus, even when reproduction is not the motive.
B: You ask about the biological sexual urge. That is impregnated in the ego mind body complex to facilitate reproduction, according to nature’s design. If the act did not involve pleasure, man would likely ask ‘Why should I give birth to children?’
     What is the highest joy man can know? It is the state of no-mind. Daily he experiences it in sleep. Only the yogi has experience of it in waking; for the Jnani there can be no question of experiencing anything. So, nature has seen fit to introduce this bliss of no-mind in the coital act.
       A moment before the commencement of the spasms that bring about emission of the seminal fluid, for precisely 1/128th of a second, the mind is removed. It is like a plate being exposed with a very quick shutter speed. The impression of the bliss tasted, leaves behind a strong craving for more such bliss in the mind. This is how men end up addicted to the act.