The beach is basically a synonym for tiredness. When you're feeling down, you go to the beach, tired and looking for a little escape, only to have a romantic encounter while you're there.
Some people enjoy being intimate outdoors, while others simply can't accept the idea at all. I do it outdoors occasionally, but I still prefer being indoors, mainly because you have more privacy. That said, being indoors does have one drawback.
You may not know this, but sometimes at the beach, the thing you've been looking forward to most and the thing you find most annoying can happen at the same time.