Just my opinion, the developer deliberately wrote her this way so that what she is doing looks like cheating (to provoke anger, jealousy, etc., in some) but she can think that she's still a good person. Porn logic, in a way. She's gullible about a lot of things because that supports the plot (David, too).
I mean, really: how can she not realize what Chris, Ron, and Marvin are up to? She has to be written as gullible so it's easier to corrupt her if that's what the player wants to do. At the same time for folks on the faithful path she hasn't really "cheated."
I wouldn't buy that argument from my wife in real life (nor would she accept it from me) but we're not living in an AVN.