Sex is never between two people.
I am not sure that is true. I could never get my wife to agree to a three way, so all of my experience is just with two. Well, I mean, since I have been married. I don't like to think back to those lonely times when the only one that would have sex with me was me.
I know you are joking and likely get my point. One of my favorite things about sex is not the typical three way nonsense or fantasizing about others. That is too obvious, but literally being with the person you with when you are with them. You double, triple, etc, the person you are with.
I am not sure how explicit we are allowed to be here, but men don't just get erections and push them into vaginas. This is nonsense. We have some fantastic relationship with the person we are with which is to say that even something like, discussing previous encounters memories, the possibility of repeating the act into the future, etc introduces multiplicity beyond duality within sex.
Sex is always reproductive in the two senses of the Latin prefix re
(for you Latinists) there is a repetitious structure in its very physical nature which is mirrored in the psyche's need repeat the act as well and a emphasizing aspect of production in sex. So sex isn't just producing something again, but something greater production in some sense.
Imagine the stupidest, simple sexual act. Even then, you have a man asking something like, do you like that? Who is he asking? Forget future and past iterations, even in the so-called present, the woman is increased. How do you answer that question as a woman? Yes?
Then the guy can ask it again: you like that don't you?
LOL! Who on earth is he talking to? Then there is the mentioning of mere current possibilities in the which the mere mentioning is the whole point. Who exactly are doing such acts? The acts are satiated as they increase arousal and yet are never engaged in but somehow feel as though they were. So who engaged in those acts?
In the end, you wife isn't your wife. She is a lot of things but she isn't your wife. And only because radically she isn't your wife is the reason she be so.
I wouldn't talk about others' wives like this as people seem to be touchy about the issue and flip out, but I hope you don't take my point as insulting even if it sounds like gibberish.