This type sounds familiar - I know a similar girl. It can be due to emotional unfulfilment - their bf or closest male friend may be wonderful but have something missing, and find it in someone else (e.g. if her boyfriend is wonderful, loyal caring, etc. but not passionate or sexually uninhibited, and someone else is much more open about being passionate). Then they will do all they can to keep both happy and sustain the relationships, e.g. with statements such as "I want to be with you, but I'm with so and so"...sounds very familiar, and often the "I want" part is what leads guys on with hope. It can also be linked to insecurities, and like you say, without being flirtatious, they may feel that it would expose their true insecurities within 'normal' friend interaction.