Mama McConaughey is dishing some serious dirt in her new book, "I Amaze Myself!" -- including how her hubby died during a romp in the sack!

"On Monday mornings, he and I often said goodbye by making love," Kay McConaughey tells Us Weekly. "One day, all of a sudden, it just happened... but it was just the best way to go!"

Surfer Matthew recently became a father to Levi with GF/handbag designer Camila Alves.

Click on an image to view the MATTHEW MCCONAUGHEY photo gallery!