Similarities and distinctions in the drug use and sexual behavior of transfeminine and nonbinary adults assigned male at birth in the United States.

Research has clearly documented high rates of human immunodeficiency virus (HIV) infection among transfeminine (TF) people (including trans women). This research has largely overlooked nonbinary people assigned male at birth (NBAMAB) despite their gender fluidity. This study examined similarities and distinctions in the occurrence and frequency of condomless anal sex (CAS) with casual male partners and associated drug use among TF and NBAMAB people. Between November 2017 and March 2020, a total of 2,758 TF and 2,347 NBAMAB adult (aged ≥18 years) participants living in the US completed an online survey assessing sociodemographics, drug use, sexual behavior, relationship status and relationship characteristics among those who were partnered. Multigroup hurdle models indicated that the occurrence and frequency of CAS with casual male partners did not differ significantly between TF and NBAMAB people who were not in a relationship. Partner gender and sexual agreement predicted CAS with a casual partner among TF but not NBAMAB respondents. Those TF participants with male partners and nonmonogamous sexual agreements had the highest odds of engaging in CAS and the highest frequency of CAS when they did. In both groups, illicit drug use was associated with higher occurrence and frequency of CAS with casual male partners. Cannabis use was not associated with either. The findings align with prior studies highlighting TF individuals' risk for HIV infection, particularly those who are single and partnered TF with male partners and nonmonogamous agreements. Despite being largely overlooked in HIV research, some people who identify as NBAMAB may engage in similar behaviors as TF individuals.