Cambodia's 100% Condom Use Programme is credited with an increase in consistent condom use in commercial sexual interactions and a decrease in HIV prevalence among female sex workers (FSWs). There has been little improvement in condom use between FSWs and non-commercial partners, prompting calls for more innovative approaches to increasing condom use in these relationships. To understand why condoms are used or not used in sexual interactions involving FSWs, we examined condom negotiation across different types of relationships. We conducted 33 in-depth interviews with young (15 to 29 years) women engaged in sex work in Phnom Penh. There was an important interplay between the meanings of condom use and the meanings of women's relationships. Commercial relationships were characterised as inherently risky and necessitated condom use. Despite a similar lack of sexual fidelity, sweetheart relationships were rarely construed as risky and typically did not involve condom use. Husbands and wives constructed their sexual interactions with each other differently, making agreement on condom use difficult. The lack of improvement in condom use in FSWs' non-commercial sexual relationships needs to be understood in relation to both sex work and the broader Cambodian sexual culture within which these relationships are embedded.