There are three kinds of sexual relationships that occur on different planes, different levels of integration between your physical, animal being, and your relational and spiritual beings. In learning about the differences between "Hook-Up Sex," "Marital Sex," and "Making Love," you can avoid the conflicts that people often experience in their sexual-romantic relationships, regardless of your age or kind of relationships (soulmate, f**k-buddy, spouse etc.)
In Hook-Up Sex you and your partner use each other's bodies for your own pleasure. It can be extremely intense and arousing, especially when you feel lust towards a new partner. There's a place for this kind of sex, but it's also the most primitive, least evolved form of sex. It reflects the purely animal part of being human -- our physiological needs and impulses. We share those with other animal species. From a human standpoint, though, it's mostly void of relationship beyond the physical connection; a form of playing through using each other's bodies.
"Marital Sex" reflects a higher plane than "Hook-Up" sex because it includes some degree of emotional connection and intimacy. At least it does at the beginning of the relationship. But what tends to happen is what this couple experienced: Their sex life became entangled with the conflicts and disagreements that had accumulated over the years. They brought all of that into the bedroom with them.
You might think of this as "spiritual sex," but I think that term is too easily equated - mistakenly -- with only ecstatic physical experience. And some recent research indicates that seeking just the experience of transcendent, physical sex can also increase the likelihood of unprotected sex. Instead, envision two partners whose sex life is interwoven with heightened mind, body, and spiritual connection.
So to summarize it all in a pinch -- "Hook-Up Sex" refers to just plain f***ing which is a purely physical encounter; "Marital Sex" is the kind of sex life that most committed couples tend to have; and "Making Love" is a different kind of experience that transcends both of the other two kinds.