I don't know that I would like to present it as a chicken/egg theory, because it then becomes some sort of tit-for-tat bartering scheme and that opens up another whole can of crap (and there is no official, unbiased scorekeeper).
I perceive it more as two people being in a place of relational security (or confidence?) and tenderness toward each other so each can express their desires/needs/longings and have those desires/needs/longings recognized as *important* AND acted upon.
But this presumes several things. That the two people are relatively healthy mentally & emotionally. That they aren't having to deal with major relational disruptions (affairs, abuse, cruelty, illness, addictions) from the past or present. That *both* individuals have the ability to occasionally step up and make the choice to "do the right thing" a large portion of the time.
People can have sex without approaching any vestige of intimacy. I can see that in a relationship that is already fairly healthy, sex *is* a conduit to intimacy. Sometimes in a troubled relationship sex can still be good, but I think it safe to say that it doesn't necessarily engender intimacy.
Long term relatively healthy relationships viewed over a period of decades can be seen as something of a teeter-totter with balance being the natural resting state.
