In marriage, a man becomes slack and selfish, and undergoes a fatty degeneration of his moral being.

In this quote, Robert Louis Stevenson, a Scottish author best known for works like Treasure Island and Dr. Jekyll and Mr. Hyde, offers a somewhat cynical view of marriage. He suggests that, over time, a man in marriage may become slack and selfish, implying that the responsibilities and commitments of a relationship may cause him to become less diligent and more self-centered. The use of the term "fatty degeneration" further suggests a decline in his moral character, likening this process to a physical deterioration of the body, which emphasizes how marriage could corrupt a person’s virtues in Stevenson’s view.

Stevenson’s choice of words like slack and selfish reflects his critique of the traditional marriage dynamic, where a partner (in this case, the man) may grow complacent or disengaged over time. He implies that marriage, rather than being a force for personal growth or mutual support, could lead to a deterioration of the individual's moral and emotional well-being. The idea of degeneration reinforces the notion that marriage might cause a man to lose his sharper sense of responsibility, ambition, and ethical values as he becomes more focused on his own comfort.

This perspective reflects a more negative view of marriage as an institution that might not necessarily nurture individual growth but instead foster a sense of moral decline. Stevenson’s critique highlights a certain disillusionment with the idea that marriage can lead to personal fulfillment or virtue. His use of terms related to physical decay underscores how, in his view, marriage could sap one's vitality, leading to a decline in moral and ethical standards.

Ultimately, Stevenson’s quote serves as a commentary on the limitations and potential pitfalls of marriage, portraying it as an institution that may not always lead to personal development or happiness. His words reflect a more traditional skepticism toward marriage, emphasizing how, rather than fostering moral growth, it might cause stagnation or regression in an individual's character.