Like Doug, I think it's about much more than adultery. One theme could be that Robert allows himself to get involved, not as a voyeur but for real. I imagine that one reason for travelling around so much and being a loner is the freedom of doing whatever you want to in such a life. Everytime someone get's too close, you just move on. For some reason (maybe age, wisdom, ...?) Robert allows deeper feelings than ever before in his life. The man who doesn't want to need anyone falls in love. He can't just move on this time without leaving something behind, he finally dared to feel! I know some people who are so afraid of loosing they never allow themselves to fall in love, because they might loose. But Robert dares, and I think this, for me, is an important theme in this story.
One other is: "we are the choices we make". This is one reason I feel only persons of a certain age can really understand this movie. As a teenager this wouldn't have meant anything to me, but now I'm married, have children, have a buisness, so my view changed. It's about not being able to do all the things you'd want to do, because of committements. A french singer (Jean-Jacques Goldman) sings this really well:
...puisq'il faut apprendre
à défaut de le comprendre
à rêver nos désirs et vivre les ainsi-soit-ils...
(My translation is not very good, it says about
... since we have to learn
even if we don't understand
to dream our desires but live life as it is...
Beautiful song!!!