We also can't rule out that being the way she is she took the affair a lot more seriously then Kirk did, particularly when he was much younger and eager to do what he needed to advance his career.
Here's another thought. Long befire TWOK was even an idea and someone conjured up the name of Carol Marcus in WNMHGB Gary Mitchell jokes about aiming a "little blonde lab technician" at Kirk at the Academy. I always wondered if it could have been Ruth (re: "Shore Leave"), but why couldn't it have been Janice Lester? She could easily have been blonde back in the day.