There are two meanings to this word (that I found anyway):
1. is the deliberate behaviour to attract attention
2. is the psychiatric condition which requires the exposure of your genitals in public.
I think with sex, there is more of the first meaning than the second, but I'm sure some of the second comes into play too.
In my wild youth, spent often in country pubs, there was a condition among the 'well hung' men to bare all when they were suitably drunk. It was only my observation, not a fact, that only well hung men did this... but I've rarely seen a smaller man give public displays of his anatomy. Have you?
So I think that some men, the well endowed kind, have a endless fascination in showing the world how much they have. Maybe this isn't a psychiatric condition, as such, but I think many times outdoor sex is initiated by the male, it's for this reason. He wants to show off his junk.
Some women have a burning desire to attract attention. And for me, they're the ones most likely to be involved in overly lusty public displays of affection. The ones who virtually start foreplay at the main bar, continue it into the beer garden and the deed is done in the carpark. Sometimes they make it home! (hope I'm not being too cynical)
Sometimes exhibitionism comes from encouraging fear - the fear of being caught. You have sex outdoors, or in a public place, or in a car in a carpark, for the fear factor. It gets your adrenalin running. It fires up the libido. Your partner gets a whole lot hotter when you're pulse is already racing.
And sometimes exhibitionism is just because you're a naturist. You like being nude. You like being outdoors without clothing. And you like having sex with freedom.
There are a lot of different aspects to exhibitionism....and I find it a fascinating topic. I'll have to write and think more about this.