To trot out those old and somewhat worn psychology concepts, the penis or phallus remains whether we acknowledge it or not, as part of our "collective unconscious". We derive so much of our modern culture, at least in the west from what Greek culture and societal norms were. Other ancient cultures also had aspects of phallus worship, and were not so prudish as modern societies are, where a phallus, especially an erect one is deemed indecent and even offensive.
I suspect that phallus worship remains somewhere in the murky depths of our subconscious in all of us. Its generative and creative powers remain a mystic secret that few of us understand fully, but nonetheless we are fascinated by it. To hold a phallus - either one's own - or another potent member, is to be somehow connected to the creative forces of the cosmos itself.
My take on this is each of us yearns to "return to the Source", achieve "union with the divine" (or any other numerous terms) - and at the moment of orgasm, there is a glimpse into the eternal creative process. One may not understand what it means, but one returns to it again and again. Is it the process of orgasm (i.e. achieving union) that we're drawn to? Or instead have we fixated on the vessel or instrument itself that conveys the "essence of life"? Perhaps the latter, being a physical object we can hold in our hands (literally), or an object we can venerate, becomes the substitute or what we end up idolising.