Young adults, including college students, engage in high levels of unprotected sexual activity despite relatively high rates of HIV/STI and pregnancy-related knowledge. Little is known about the cognitive strategies that young people use to explain this inconsistency. The current study examined young people's explanations for engaging in unprotected sexual activity in their committed relationships. A total of 63 young adults (32 women and 31 men) completed daily diaries over a 3-week period, providing a total of 1,284 daily reports tracking their condom use and non-use during intercourse. Diary collection was followed by in-depth interviews designed to explore participants' decision-making regarding their participation in sexual intercourse unprotected against infection or unwanted pregnancy. Less than a quarter of the sample used condoms or oral contraceptives consistently. Participants primarily viewed condoms as a means of preventing pregnancy; few described disease prevention as a main motivation for their use. Analysis of the cognitions underlying explanations for condom and contraception non-use were classified as (1) general biased risk evaluation, (2) biased evidence evaluation, (3) endorsement of poor alternatives, (4) focus on spurious justifications, (5) dismissing risk, and (6) ignoring risk. Prevention interventions should incorporate methods to challenge young people to acknowledge personal risk and commit themselves to taking steps to reduce this risk.