I think "casual sex" is unrealistic in that sexual interactions among we humans are not so insignificant as that: not physically nor emotionally. And if sexual interactions really were that insignificant, what would be the point, really? Masturbation would be far more precise and efficient.
And because the idea of "casual sex" is so unrealistic, people get themselves in all sorts of contention and difficulty by acting on the presumption that it is realistic. I don't think casual sexual intercourse is "immoral", I just think is an unrealistic idea, among humans. For many animals, yes, but not for the vast majority of humans.