The problem with casual relationships is a language-problem.
We tend to assume, even before any reflection is possible, that casual relationships are defined in opposition to serious relationships. This, as you can understand for yourself, is a misunderstanding.
Let us consider how we process the word ‘casual’.
Casual relationships, if you want an oppositional definition, are the opposite of monogamous, codependent and marital relationships. Casual is not an antonym of serious.
Casual relationships, just like any other kinds of relationships, can be equally or more serious. It is entirely possible and even advisable to be serious in all sorts of relations including casual ones. They are, and should be, serious. The sex, the intimacy, the feeling, the trust, all of these must be serious. Otherwise it is just a shell of a relationship.
The mass of earth-dwellers think that ‘casual’ implies a lack of seriousness, a lack of trust, of emotional safe haven, of care. This is an utterly misplaced, if not to say childish and rather unreflective, idea of the casual. In which language is the word ‘casual’ antagonistic to the word ‘serious’? The earth-dwellers, in the din that they create themselves, cannot be bothered to think about such fundamental stuff such as meaning. But in the expanse of thought afforded by certain orientations of oneself, the din and clamour disappears and reality is instead illuminated by the slow and steady glow of recognition.