I've just come back from a week in St Ives and the nice lady in the Tourist Information office recommended Porthkidney beach dunes.  There were quite a few people there, mostly single men, and a lot of walking around, it has to be said.  However, I spoke to a couple of the locals and they said that the etiquette there was to stay in the dunes, not on the beach, and to wear trunks or something if you had to stand up or walk about.

I have been to Vault beach near Gorran Haven it is quite a good beach. The far end is the nude bit. Also Porth Kidney beach just outside St Ives is a great beach.