So the movie is basically him learning about this, and coping with it. It's kind of devastating in a way, but also fascinating. From what we can tell, his parents never actually cheated on each other, and were committed to staying married, even though it wasn't very happy. At moments they seemed kind of resigned to making the best of it, which was at least an improvement from the moments of seething misery, but still.
Thinking back on it, one does wonder whether his mother doesn't ultimately bear a bit too much of the blame for it all. Though he insists that he loves her, the filmmaker ultimately makes her seem like a rather narcissistic, melodramatic woman who is never especially warm or loving to her husband or children. His father, who had formerly been a rather aloof and taciturn type of guy, turns into a cuddly teddy bear after his second marriage. And so you can't help but think, my god, all those years he suffered. Though of course the same could be said of his mother. And in the end you wonder, was it really worth it, to stay together all that time? Which also makes you wonder, more broadly, what marriage is really about, and how happy two people really can be together. I mean, we know that all marriages have their ups and downs and difficult points, but how much misery is too much, you know? When do you just give up?