It's called "Perineum Sunning" and doctors hate it.
A number of naturalists, yogis, interesting people have been touting the benefits of just letting the sun tickle your bum. And doctors are now having to express how terrible of an idea that actually is. Or, at the very least, they express the need to use sun protection in the form of clothing or sunscreen on ANY body part that you're exposing to the sun.
Put some zinc on it? That's fashion.