Nearly every boyfriend I've had has taken sexual advantage of me in some form, at some point. Generally these incidents were right on the borderline of what would be considered assault, and one outright ignored consent.
I am very picky and only date people I've known well and trusted for a long time - this was true even before these things happened. These people did not show any red flags for this sort of behavior until it happened... they became aroused, and suddenly were different - not caring anymore whether or not I was okay with what was happening. They just wanted what they wanted.
I know that this is not true of all people who happen to be male, but the fact that it's happened so consistently, with people who were nothing but empathetic and respectful up until that point, makes it really damn hard to not assume the worst in a man's intentions when considering him as a potential partner.