Men who have sex with men (MSM) remain at high risk for HIV infection. Culturally specific sexual identities, encompassing sexual roles, behavior, and appearance, may shape MSM's experiences of stigmatization and discrimination, and affect their vulnerability to HIV. This multi-site qualitative study (n = 363) encompassing 31 focus group discussions (FGDs) and 121 in-depth interviews (IDIs) across 15 sites in India investigated sexual identity formation, identity practices, and transitions and their implications for HIV prevention. IDIs and FGDs were transcribed, translated, and underwent thematic analysis. Our findings document heterogeneous sexual identity formation, with MSM who have more gender nonconforming behaviors or appearance reporting greater family- and community-level disapproval, harassment, violence, and exclusion. Concealing feminine aspects of sexual identities was important in daily life, especially for married MSM. Some participants negotiated their identity practices in accordance with socioeconomic and cultural pressures, including taking on identity characteristics to suit consumer demand in sex work and on extended periods of joining communities of hijras (sometimes called TG or transgender women). Participants also reported that some MSM transition toward more feminine and hijra or transgender women identities, motivated by intersecting desires for feminine gender expression and by social exclusion and economic marginalization. Future studies should collect information on gender nonconformity stigma, and any changes in sexual identity practices or plans for transitions to other identities over time, in relation to HIV risk behaviors and outcomes.