Thanks for the interest and the thoughtful question.
Social is, at root: bonding, warmth, interpersonal intimacy, relationships, love between two people, friendship, having each other's back. It is also the human need that lies behind the sentiment of loneliness. (Any instinct type can feel lonely.)
If you think about it, not all your close relationships are sexual. You might have a one-on-one intense bond with your brother or sister, or one of your parents, or perhaps your teacher or boss who is elderly when you're a child; but none of these bonds (under typical healthy conditions) would be sexual. You can bond deeply with your sister without there having to be 'sexual charge.' So what would you call your intense, trusting, loving relationship bond with your sister? That would be social. And this extends to your friends.
Sexual is, at root: heat, allure, transformation, sexual intimacy eros. There's a sense that you want to penetrate and be penetrated by the other person entirely, as though being absorbed into their being; tearing down all walls. (This is often mistaken for intimacy, but it isn't necessarily, unless the Social instinct is also at play.) It is also the human need that lies behind obsession, limerence. (Any instinct type can experience this.) There's an addictive quality to it, whether or not you're actually intimate with the person.. you want them to want you.
In French, orgasm is called "la petite morte" - the little death. This is because when making love, you're naked, exposed, without walls. You spill your life seed into, or upon, another. Fluids are mixed. Boundaries are lost, and when boundaries are lost, it's impossible not to transform; to be reborn.
The reason sex has been deemed 'sinful' and bad is because it's too destructive to society - it brings unexpected changes. People who were otherwise loyal to their family, or their job, or "the state" - will suddenly throw it all away for the sake of passion. Or that is what they fear. Sx instinct has a transformative quality.. it is there to strip you down, expose you and entice you, leaving you wide open. In this sense, Sexual Instinct has a danger to it. It can be scary, overwhelming.
Sx dominants are tuned into enticement, allure; they can't turn it off. There's a sense they're always penetrating into you, using some type of luring siren signal, like the way birds show off their bright feathers and sing to impress a mate. All of that is sexual signaling and humans do it very similarly - through dressing up, showing virtuosity to entice (music, art etc).
Now this does not mean that "Sx dominants are sluts." Quite the contrary, in many cases. The Sx dominant is so deeply attuned to chemistry that they can tell whose chemistry mixes best with theirs. When this instinct is first, it can be very selective, holding out for the hottest person, some kind of Ideal Other who would attract them and allure and entice them for all eternity.
This can, of course, develop into a sense of intimacy very quickly- since there's an addictive quality, wanting to get deep into every part of the lover's psyche. But this is a very different type of intimacy than that which you experience with your sister. The type with your sister will outlast most of your sexual relationships. The intimacy with your lover is more penetrating and intense, but it is not in and of itself based on common interests, trust, deep bonding and so forth; until Social instinct comes into play.
And we must remember that people are whole - we are not "just one instinct." So an Sx/Sp and Sp/Sx couple will become bonded on a social level. Just as an So/Sp and Sp/So couple will enjoy heat and intensity, and want to allure each other. The question is, where does your attention automatically lie? What is the primary call of your instinctual senses? Which instinct is on all the time, in all situations, constantly guiding you, alternately holding you back and propelling you forward?