So his marriage/life was in a bad place and he was feeling detached, and the connection with Charlotte turned him on (as in a person with feelings again), yet the respect and importance disallowed the sexual connection with her, but it had him buggin so he went with someone else to ensure the preservation of continued good sense with Charlotte. Though inasmuch it also showed some flaws in himself.
Then her reaction also shows that she was feeling the tension as well, and was a little upset about him not chosing her, though (because she was younger) maybe not understanding the reasons why he did with the singer lady instead of her. However them getting over it later on could show she had an understanding of his actions after having some time to think about it.
And for the plot it demonstrates the reality of how serious their relationship was (if only because it was based on a real heart connection) and how because it was of such a nature, it couldn't have lasted (or at least stayed healthy) even if geographical circumstances allowed otherwise.
Then also since the movie is about how their friendship reawakened them to life and living and relationships and gave them hope for the possibilities of better things (if they applied what they learned) in the lives they were returning to, then bringing out this point of intimacy more directly points out the possibility of renewed intimacy with their spouses.
Thanks for the input guys, you got my gears turning and I like the movie even more now.