I worried that I was going to have to be primarily a writer. Why worry, you might ask? Well, although it is true that one feels fantastic when one has finished a writing task, it is mostly horrible while one is doing it. You will see therefore that writing, ghastly at the time but great afterwards, is exactly the opposite of sex. All that keeps one going is the knowledge that one will feel good when it’s all over.

Stephen Fry (b. 1957) British actor, writer, comedian

The Fry Chronicles: An Autobiography, Part 2 “Comedy” (2010)
    (Source)

Often paraphrased: "Writing is ghastly at the time, but great afterwards, exactly the opposite of sex."