Existential Sex

Isn't the existential concept of "authenticity" absolutely essential to meaningful, long-term relationships? Not that sex has to take place solely within meaningful, long-term relationships, but I suggest that it's possible to have a meaningful, long-term relationship and still have reasonably good sex. Or am I just being a '70's reject?

On a related note, I know I'm the last person in the English-speaking world who understands the concept of "reciprocity," but I am convinced that reciprocity is essential to ANY kind of intimate relationship. The notion that the "relationship" is all the doing of one person and has nothing to do with the other is, all things being equal, invalid and probably an excuse for one person to play it both ways. One partner has "responsibilities," the other has "freedom." Reciprocity is only one argument for why such a subtext is fallacious (and, BTW, inauthentic).

These concepts seem immediatly obvious, but they are so removed from the subtext of "modern" discussion regarding intimacy that I feel like I really missed a memo at some point. Has there been some sort of definitive breakthrough regarding what human relations are "really" about? Or has it been just another case illustrating teh prrinciple that if a thing is worth doing, it is worth doing badly? Was there just a general realization that while it is difficult to conduct relationships well, it is facile to conduct them poorly?