Young adults are disproportionately affected by negative sexual health outcomes, such as sexually transmitted infections. One strategy for reducing such negative outcomes involves the use of comprehensive sexual health education. As comprehensive sexual health education programmes are adopted, there is a need to evaluate the messaging approaches that might best connect with young people. Sex-positive approaches, which describe sex as a healthy practice and personal choice, are becoming popular, but whether this strategy works has not been tested from a message design perspective. This study examines the reactions of young adults to sex-positive or sex-negative messages, framed as a gain or loss, designed for a text message-based intervention. Participants quantitatively evaluated 24 messages on their mobile phones. Gain frames were preferred over loss frames, and sex-positive messages were rated as more believable and persuasive. An interaction between the two concepts was also found. Results highlight how sex-positive messages that focus on the benefits of certain behaviours tend to resonate better with young adults. Implications for health educators and practitioners are discussed.