Let us never underestimate the joy of spontaneous sex. How the rush of it all makes us feel and how desirable it leaves us. There are so many ways to spice up your life and this is just one of so many!
Well, you may get something spontaneous on your privates, too. I think, while the idea tickles some of our fancies, it can be more bad than good to do so, unless you know for sure that stranger is a dork who doesn't get much action and is less likely to have an STD. Even then, not a good idea. They could be a serial killer, the list goes on!
Gomer Pyle to woman, "Can I put my finger in your belly button?" Woman to Gomer Pyle, "I guess." Woman to Gomer Pyle, "That's not my belly button!" Gomer Pyle to woman, "Suprise, Suprise, that's not my finger!"