Bass' publicist, Ken Sunshine, didn't respond to a message from The Associated Press. The AP was trying to locate a representative for Lehmkuhl.
Bass, who was part of the boy band 'N Sync, revealed earlier this year that he is gay and was in a relationship with Lehmkuhl, a former Air Force captain and winner of season four of CBS' "Amazing Race."
The 27-year-old singer told People in July that he didn't earlier disclose his sexuality because he didn't want to affect 'N Sync's popularity.
"The thing is, I'm not ashamed — that's the one thing I want to say," Bass told the magazine. "I don't think it's wrong, I'm not devastated going through this. I'm more liberated and happy than I've been my whole life."
'N Sync, known for a string of hits including "Bye Bye Bye" and "It's Gonna Be Me," went on hiatus in 2002. Bass has also made headlines for undertaking astronaut training and failing to raise money for a trip into space.
Lehmkuhl, 32, has said he admired Bass' decision to disclose his sexuality. Lehmkuhl has a new book, "Here's What We'll Say," which recounts his time keeping his sexual orientation a secret from Air Force colleagues.