One reason, illustrated by this wonderful sex tip from Cosmo:
THE SENSUAL SCENARIO
You're headed for a hike and a picnic with your honey, but little does he know, this is no ordinary day in the great outdoors. You'll bring out his back-to-the-wild side with in-the-mood foods and surprising tools.
PASSION PROPS
Soft blanket; iced tea; citronella candles; a clean, smooth rock; tangerines; extra-gentle baby wipes; citronella-containing lotion
SMOOTH MOVES
1. Open the blanket and light the bug-repelling candles.
2. Massage your bodies with the lotion. Don't apply to your breasts or from his belly button to his penis—the lotion doesn't taste great.
3. Take a tangerine and squeeze the juice just below his navel and slowly lick it off, then ask him to do the same to you. (Use baby wipes to clean up any stickiness.)
4. Get in the missionary position. Use the rock to gently rub the area just behind his testicles at the moment he's about to climax.
(I couldn't find the original link to Cosmo but it's referenced enough that I suspect it was from the pre-internet days.
My source.)
Edit:
Additionally.