In this study, we examined how beliefs about the nature and origin of sexual orientation were associated with sexual identity outcomes, namely internalized sexual stigma and sexual orientation uncertainty, and in turn, psychological well-being in sexual minority women. A community sample of 393 lesbian and 205 bisexual women were recruited for a cross-sectional online survey. Using multigroup structural equation modeling, we examined whether believing sexual orientation to be inborn/immutable (i.e., natural) and/or as existing in discrete categories (i.e., discrete) was indirectly associated with psychological well-being via internalized stigma and sexual orientation uncertainty and whether some of these relationships were moderated by nonprototypical attractions (nonexclusive same-sex attractions in lesbian women and straight- or lesbian-leaning attractions in bisexual women) and age. Overall, similar patterns of direct effects were observed in lesbian and bisexual women. In both groups, naturalness beliefs were associated with lower internalized-stigma, whereas discreteness beliefs were associated with greater internalized stigma. In bisexual women, nonprototypical sexual attraction patterns moderated the relationship between discreteness beliefs and sexual orientation uncertainty. Younger age was associated with increased sexual orientation uncertainty in both lesbian and bisexual women. Specific to lesbian women, nonprototypical attraction predicted less endorsement of immutability and binary/discrete beliefs about sexual orientation, and unexpectedly, greater internalized stigma. The present study identifies potential implications of common lay theories of sexual orientation for lesbian versus bisexual women.