Individualism
The decision not to marry is a presumed consequence of Søren Kierkegaard's philosophy. His well-documented relationship with Regine Olsen is a subject of study in existentialism, as he called off their engagement despite mutual love. Kierkegaard seems to have loved Regine but was unable to reconcile the prospect of marriage with his vocation as a writer and his passionate and introspective Christianity.
A similar argument is found in Franz Kafka's journal entry titled "Summary of all the arguments for and against my marriage":
I must be alone a great deal. What I accomplished was only the result of being alone.
As a high-profile couple, Jean-Paul Sartre and Simone de Beauvoir always expressed opposition to marriage. Marriage, understood existentially, proposes to join two free selves into one heading, thus denying the freedom, the complete foundation, of each self.
Read more about this topic: Criticism Of Marriage