Michael J. Fitzgibbon, 37, also pleaded no contest to more serious sex charges involving sex acts with a 12-year-old girl, a relative.
Authorities say Fitzgibbon's girlfriend was afraid of losing him while she recuperated from gastric bypass surgery, so she arranged for him to have sex with her daughter for two months.
All three of them allegedly signed a contract that allowed the girl privileges such as piercings and hair dye in exchange for the sex acts. She testified earlier that she and Fitzgibbon had sex two to three times each week for two months last summer.
Fitzgibbon faces sentencing for the six counts of criminal sexual conduct on March 23. A no-contest plea is not an admission of guilt but is treated as such at sentencing.
The girlfriend, whose name is not being made public to protect her daughter's identity, is scheduled to stand trial in February on three counts of criminal sexual conduct.
Authorities investigated after the 15-year-old told a relative in July. The 12-year-old came forward after charges were filed.