When I first played ME1 with Molly, I talked to Kaidan and listened to his blah blah blah and, while I found his history interesting, I hated that my simply listening to him equated to flirting, despite not picking the obvious romance options. I was very intent on sacrificing Kaidan on VIrmire.
One of the reasons for creating Tarisa was that I suddenly became interested in Kaidan and wanted to play out the romance. I wanted to give him a chance. Initially, my bias was still there, but I quickly softened to him and Tarisa has stayed true, though it hurt her deeply to have him turn his back on her on Horizon. Fortunately, he sent that email shortly after, and she kept it close and successfully resisted Garrus’ calibrations and Thane’s “siha”ing. I started up Tarisa’s ME3 runthrough JUST to get to her reunion with Kaidan, after watching Molly’s reunion with Ashley. I loved every moment of their discussions on Mars, especially her requesting his trust.