Sex is a frantic, grunting, sweaty thing. Small boys try to imagine it, little girls romanticise it, teenagers crave it, adolescent males are incapable of thinking about anything else, grown men wax nostalgic about it, mature women fantasize about it and nobody is getting as much as they would like - in short, it's the single biggest preoccupation of the entire human race.
How can you call that casual?