It comes from personal experience. People feel like everything else is treatable so if HIV is prevented than they’re all good to take off the condom. Can’t tell you the number of times I heard this from friends about their own behavior, from people trying to have sex with me, and from others trying to have sex with my friends (condomless).
Also: https://www.ncbi.nlm.nih.gov/m/pubmed/29509889/