How desperate was this woman?

As actors I enjoyed performances by both and they were very good together as a team. However, I think that he should have been honest, divorced his wife and married Hepburn if they were so enthralled with each other. His marriage apparently had no more significance to him than an outdated legal contract. Honesty is always better than living a lie and less painful in the long run.

I neither know, nor care much about their personal life, but they made some memorable movies together.

Perhaps, but Spencer Tracy was a devout Catholic and never divorced his wife. They had a marriage like a lot of them before divorce was common - perhaps loveless and dead, but legally intact. Hepburn brought him real happiness and he brought her real happiness. They were great together.