Norwegian Wood

WHY did Watanabe have to sleep with Reiko in Haruki Murakami's book "Norwegian Wood?"
WHY?WHY?
I just finished the book, and fine, I get it .. he needs to sleep with random girls to dull his longings, he needs to sleep with Naoko because he loves her, and he needs to sleep with Midori because he's IN love with her, but Reiko???
Here, the book had a change to describe a "deep and meaningful" relationship with depth and insight and then it ALSO turns this relation into sex again? WHY? Does all transitions in a mans life need to be defined by sex? I was upset. Of all the things I got from that book, Murakami torn it down by making it necessary for Watanabe to have sexual relations to all the female charcters. What happened to the acknowledgement of the meaning of friendship... I hated that turn... As mush as I loved some of the quotes in thw book. e.g "her cry was the saddest sound of orgasm I had ever heard" [p. 40]