The pleasure of outdoor sex/sex in public, is of course not only the fantastic feeling of the sun's rays warming one's naked abdomen, and the gentle breath of Mother Nature, caressing one's body. To a large extent, it's the exhilarating risk of being discovered by random passers-by, or the idea of someone could lurking from behind the nearest bush or tree, or from a distance, with binoculars.
Because we're well aware, that once the ecstasy kicks in and climax is building up, our senses, that previously "stood guard", are reduced, because we only have each other's bodies and pleasure in mind. But our subconscious remembers that, and thats what makes outdoor/public sex, into something absolutely unique and gives the best orgasms in the whole world.