Yunior ends up getting caught for one of the most socially unacceptable types of mistakes—cheating on one's girlfriend. For this, he ends up watching from a distance as Magda deals with those emotional consequences. Betrayed by his affair, betrayed by one of the girls to the other, Yunior must simultaneously watch the damage he caused while experiencing the confusing, shameful emotions of his mistakes. What do these things matter in light of Rafa's story though? The parallel is that Yunior is learning how to treat people by making mistakes and suffering little "deaths," so in light of Rafa's real death, although the emotions seem powerful, they are not really that important in light of problems like cancer.
That doesn't mean that Yunior is guiltless, nor does it mean that guilt even needs to be assigned. But, at the end of the day, Yunior's close contact with death adds a level of confusion to his romantic mistakes, because what is the point of it all anyway? The point is to create families, not just to acquire approval and affection. Therefore, Yunior stands in his own way as his own singular obstacle. In order to overcome his propensity to rob girls of their love in a predatory, treacherous way, Yunior must address his fear. Death helps him anchor his fear on the true object of that fear—his body fears its own end, and he is using girls as a coping mechanism to that fear and his fear of powerlessness.
In light of the novel's title, a thesis might become evident. This Is How You Lose Her? How is that? And how to lose whom? Well, that's the irony of the title, and since there is this irony, the reader might consider the thematic importance of it—not only does Yunior lose Magda, he loses all his girlfriends, in a slow, nightmarish way, over and over again, because he doesn't have the experience or bravery to commit to a girlfriend and convert her to being his wife as an adult. Instead of settling down with one girl, he continues cheating and loses out on whom? His own wife. He loses out on the benefit of marriage.
