He reportedly was never in a relationship. This comes from unidentified members of the Ritchie family who set up a web site focussed mostly on his unpublished PhD thesis. The site includes some personal information on his life.
From https://dmrthesis.net/story-and-question/ (near the bottom)
"In his adult years Dennis had a reputation for being famously private.... There were topics that were completely off limits with Dennis – mostly having to do with any form of personal intimacy. This was true for his Bell Labs colleagues, who speak of Dennis’ personal life as a kind of private zone where they do not tread, and true with his siblings also. This was just his way.
"As an adult, Dennis lived an ascetic lifestyle. In September 1967, he moved back into the attic of his family home and set up a home office in the basement, where he lived with his parents until 1989. He worked from 1PM until 3AM six days a week, Sundays off for reading. He had no friends outside of his business colleagues, he rarely socialized, he did not have (nor never had) a relationship with anyone, he wouldn’t talk about/acknowledge discussion about emotional issues in any form. (He did have a wonderful family who loved him dearly and whom he loved dearly as well.) It was almost as if he needed to blot out any awareness of his personal life from the world by using the strategy of having no personal life to be observed.
"This affect wasn’t just something neutral, it could be an active force."