An erect penis was a good lock symbol.
That’s what she said!
I think it also indicated the importance of fertility - or rather, the generative power in nature - which also relates to one of our interpretations of the ‘Sheela na gig’:
http://en.wikipedia.org/wiki/Sheela_na_gig
There are still places in Asia, where people carry around a small replica of a penis - such as the ‘Palad khik’ in Thailand - for protection from certain things, or help toward others.
These things - reverence for the concepts - are ingrained in us from very ancient times.