Re: @Dr Dan
Just because homo sapiens have bigger penises than our evolutionary ancestors doesn't mean it is a survival trait. It might have been, but could have stopped being so somewhere along the way during the 10 million or so years since we branched off the last common ancestor with modern day primates.
Before homo sapiens began wearing clothing, each man's penis size would have been known by everyone else in the group, so it would be possible for females to select upon - if indeed they had much say. It seems more likely to me that bringing home the bacon (or mastadon) or fending off the saber toothed tiger is what really made the cavewomen swoon.
Just like with human male penis size compared to apes, the same situation occurs with respect to human female breast size being (typically) much larger than apes.
The theory I have heard for this latter is that apart from humans, most mammals - even apes - still have a bent (as opposed to fully upright) posture if not entirely 4-legged. And in this physical situation, the most obvious sexually attractive feature are the buttocks.
Therefore, as hominids developed an upright stance, the buttocks became less visible and the human female breast grew bigger to appear buttock-like. Breast cleavage and buttock cleavage look quite similar.
Therefore, if the above theory holds any sort of water, the same could be true for males with respect to the penis. As you rightly pointed out, clothing and covering of the genital regions in hominids is a tiny, tiny part of our evolution history since becoming upright. Therefore as we developed a more upright stance, females visible sexual attributes moved from the buttocks to the breasts, and the male penis became more prominent, more visible, and perhaps likewise grew to become the primary visible sexual attribute.
Of course, this could all just be shit.