Hmmmm. This is a hard one for me. Honestly, I have been on a lot of beaches, I love the beach, but I'm not sure I could have sex on one. I'm not fond of the little beach hoppers that are scavanging the fresh beach-kill in the pack sand, maybe on a rock beach, where we could take shelter behind a large boulder ^.^ or with a really really REALLY big blanket. I'm weird about bugs and grimey stuff.
Edited to add: Wait, do lake beaches count? Because I've totally done that, and would again, lol. Not near as dirty a lot of the time. Also, noteworthy, I would completely do it in high dunes. Easy to sneak off, ocean waves crashing near by, but far enough away from critters to feel comfy. Came very close to doing that at Oregon Dunes this year on my annual road trip, but chickened out.
however! In the spirit of conquering national landmarks with sexual deviancy, I DID get it on at Crater Lake!