Background Our previous research in San Francisco's Mission District, a predominantly Latino neighbourhood characterised by high immigration, poverty, and gang-related violence, highlighted diverse partnership patterns among youth and the role of gang affiliation in high-risk sexual activity and pregnancy. 38% of Latino youth reported concurrent partnerships.

Methods We conducted 33 semi-structured in-depth interviews with immigrant and US-born male and female Latino youth aged 16–21 to explore how the social environment affected partnership formation and STI risk. Participants were recruited from community venues; interviews were conducted in Spanish or English. Interview transcripts were coded in Atlast.ti. We analysed the following themes: relationship fidelity; STI prevention; and concurrency norms (agreement with the statement, Many Mission youth in a relationship are also seeing someone on the side).

Results Participants were 48% female; had a mean age of 17.5 years; and 55% were immigrants from Mexico or Central America. 27 of 30 believed that concurrency is widespread, described as “cheating” (having a “main girl with ‘friends with benefits’ on the side”) or resulting from initiating new relationships before ending existing ones or between serial break-ups with a main partner. Infidelity was motivated by machismo, a cultural notion of masculinity that was heightened for men respected for having multiple partners; boredom with a single partner; an ideal that fidelity was only relevant within marriage with cheating expected prior to that; and revenge for known, suspected or pre-emptive cheating. Street gangs introduced distinct reasons for young males preferring multiple, casual partnerships: limited time for relationships because of obligations to the gang first and foremost; disinterest in committed relations to avoid emotional vulnerability given emotional demands of street life; and multiple partners bringing status: “you got more game; you got more reputation.” Young women recognised that having partners in a gang brought social prestige and articulated strategies for increasing intimacy (eg, pregnancy) to draw partners away from the street and strengthen commitment to the relationship. Despite a perception of pervasive concurrency, STI risk perception was low.

Conclusions Many relationship norms and expectations held by youth supported concurrency, highlighting the importance of addressing social influences like gangs in STI prevention.