Brian did not tie up his past relationship before arriving to the mansion. He even went as far as telling his girlfriend that they should take a break until a particular date, which also happened to be the return date from filming The Bachelorette.
Desiree gave him one last chance to come clean before his girlfriend arrived at the mansion to confront him about the situation but he chose to continue with his lie. Do you love or hate him? Scratch that, how much do you hate him?