In-depth interviews were conducted with 23 sexually-active adults receiving antiretroviral treatment (ART) in Mombasa Kenya to understand changes in sexual behaviour after treatment initiation and factors influencing condom use. Advanced HIV disease had previously led to marked decreases in sexual desire and function. After HIV testing, numbers of partners reduced and monogamous relationships began to predominate. Receipt of ART strengthened these changes, while improving sexual health. However, concurrent sexual partnerships continue within polygamous marriage and unprotected sex occurs with regular partners, even those who are HIV-negative. Those who used condoms inconsistently prior to ART often remained inconsistent users thereafter. While disclosure of HIV status appeared to support condom use, this does not always predict protected sex. In addition to classic perceptions about condom's effect on intimacy and trust, traditional gender roles, misconceptions about potential harm from condoms and fertility desires hinder condom use.