Marriage is better than seduction.
Marriage is a huge issue in this book, because the central argument (Aestheticism versus ethical wisdom) essentially boils down to a simple issue: Johannes Climacus (a fictitious reference to Kierkegaard's own life) keeps breaking up with girls because he confuses their entertainment value for his love.
The reason The Judge says marriage is better than seduction is that there are future goals that are worth pursuing, and what the older, wiser character knows that Climacus does not is that life brings more than just pleasure, entertainment, and bliss; life brings challenges and pains like the deaths of one's parents, issues that are way easier to endure with a loving spouse, but Climacus (because he cannot see the future) remains unwilling to open the letter and commit himself to ethics (which in this case means marriage).
Ethical wisdom is more valuable than pleasure or entertainment.
The central argument of this book (which could best be described as a thought experiment) is that as a youthful kid, the bachelor in Part One is evil. He is evil because he sees the world as an entertainment system designed to give him pleasure, which makes him an abusive fiance (who dumps all his fiancees once the thrill of the chase is gone).
The Judge offers him a new path: Ethical wisdom. Instead of focusing on the way experience feels, The Judge invites him to consider that the aesthetic value of experience might be pointing to deeper aspects of life, like the sacrificial aspect of love, or the accomplishment of a skillful marriage.
Humans are being judged for their lives.
There is a karmic element to this book because the second half of the letter is written by "The Judge," leaving the reader to conclude for themselves what exactly that title signifies. One obvious answer is that "The Judge" represents the higher self, like Christ-consciousness (in Christianity) or Buddha-consciousness (in Buddhism). Another answer might be that "The Judge" is a metaphor for God.
In any case, it is clear that Kierkegaard views the sins of the bachelor from the position of divine recompense. Another way to say, "You should be ethical," is to say, "Consider that maybe you're being judged."
