Katharine Hepburn noted many times she never asked nor wanted Spencer Tracy to leave his wife and marry her; Their romance was enough for Her.
Katharine Hepburn also noted that Louise Tracy by developing the John Tracy Clinic for the Deaf was a Noble person.
I believe they really had different attitudes back in the 30s-60s. I mean seriously, she and Tracy cohabited for the last 25 years of his life, so for all intents and purposes and in all but name, he HAD left his wife for her. He just didn't get a divorce and Hepburn evidently did not feel the need for marriage.
I love the story that, after Tracy died, Hepburn called his widow (I wonder if it was the first time they ever spoke) to say that perhaps they should talk (both grieving for the same man, etc.) and Mrs. Tracy's reaction was, "I thought you were a rumor."