Misato: Tell me Shinji. Do you want to become one with me? To be of
one mind and body? It could be very very nice. Ask me anytime,
Asuka: Hey Shinji, idiot. Don't you want to be one with me? To be of
one mind and body? It would be really awesome. You oughta'
appreciate your good fortune, come on.
Rei: Ikari. Do you want to become one with me? To be of one mind
and body? It could be very pleasant. Shinji.
Misato: Do you want to become one with me?
Asuka: To be of one mind and body and soul?
Misato & Asuka & Rei: It could be very pleasant. Relax and release
I've got a few theories on it. Romantic involvement seems to be a logical explanation to this line of questioning, as it is coming from the three important women in his life, however after being in Eva for a month gradually slipping away from reality, it would seem random to start thinking about your love life. Also, at this point it's doubtful he would be felling anything romantic towards Misato or Rei. So what does it mean to become one with them. It's implied that theres a choice to be made here, and each of the outcomes is unique based on the character's personality: Rei is relaxing, Misato is nurturing (or perhaps sexual pleasurable), and Asuka is exciting.
So other possible explanations for the scene could be foreshadowing third impact, but that would seem out of place as well, as Shinji would no nothing about it at this point. My current tentative theory is that this relates impart to the women in question, and impart to Shinji's personality. It's related to Jung's concept of the Anima, the female part of Man's Psyche http://en.wikipedia.org/wiki/Anima_(Jung). One of his collective unconscious archetypes, the anima is reflective of a man's view of women, and the repressed feminine qualities of his personality. In a nutshell, even if Shinji didn't go back he would still have to decide who he was going to be with, and that dessision would profoundly affect him and his future.
TL;DR- What do you make of this scene.