Few data exist on the early sexual behavior patterns of contemporary young men who have sex with men (YMSM), the social context of these patterns, and which of these factors influence risk for HIV and other sexually transmitted infections (STI). We enrolled 94 YMSM (age 16–30) into a 1-year cohort study with serial online retrospective surveys and HIV/STI testing. The first three partnerships of YMSM were characterized by relatively high rates of unprotected anal sex and a rapidly expanding sexual repertoire, but also increasing rates of HIV status disclosure. During follow-up, 17 % of YMSM reported any nonconcordant unprotected anal intercourse (NCUAI) and 15 % were newly diagnosed with HIV/STI. Sex education in high school and current maternal support were protective against HIV/STI, while isolation from family and friends was associated with recent NCUAI. Social support—including from parents, peers, and school-based sex education—may help mitigate HIV/STI risk in this population.