Research on relationship quality in same-sex couples has rarely focused on (a) couples who are parents, who likely experience additional stressors, or (b) couples in which partners differ in sexual identity. Insomuch as nonmonosexual women (i.e., women with non-exclusive sexual orientations) experience unique challenges due to monosexism, relationship quality may be influenced by whether partners share a monosexual or nonmonosexual identity. The current study is a longitudinal, dyadic analysis of 118 female parents within 63 same-sex couples whose relationship quality (relationship maintenance, conflict, love, ambivalence) was assessed at five time points across the first 5 years of adoptive parenthood. Monosexual women were those who identified as exclusively lesbian/gay (n = 68); nonmonosexual women were those who identified as mostly lesbian/gay, bisexual, queer, pansexual, or mostly heterosexual (n = 50). Analyses revealed both actor and partner effects on maintenance and conflict, such that nonmonosexual women reported more maintenance and conflict than monosexual women, and women with nonmonosexual partners reported more maintenance and conflict than women with monosexual partners. Depression was related to greater conflict and ambivalence and less love; internalized sexual stigma was related to greater conflict and ambivalence. In terms of change over time, maintenance and love declined whereas ambivalence increased during early parenthood.