Jesus said, “From the beginning of creation, God made them male and female. For this reason a man shall leave his father and mother and be joined to his wife, and the two shall become one flesh. So they are no longer two, but one flesh. Therefore what God has joined together, let no one separate.”
Jesus taught that sexual activity has a purpose. It is to be used to unify a man and a woman who have been joined together by God.
The definition of “casual sex” provided for this question is as follows: “Sexual activity that takes place outside a romantic relationship and implies an absence of commitment, emotional attachment, or familiarity between sexual partners.” That is pretty much the exact opposite of what Jesus taught.
When comparing the two, the teaching of Jesus regarding sexual activity seems to me much more human, fulfilling, admirable, and even desirable.