Everyone here has made great points, however the age range of which you are generalizing women has some big distinctions. There are 2 mentalities that can be generalized from 2 seperate age groups.
16-19 (High School Girls)
The girls in high school who are in relationships, are very much not likely to cheat in any way shape or form. Some won't even hang out with guys alone without their boyfriend present. The reason being, girls in high school are very much looking for love. They want that perfect relationship that will last forever. Mind you this is the general mentality. There can be many variables to alter the situation. In some cases, a select group of girls will be very promiscuous. Which in most schools, these are the girls who are labeled as sluts. As to the girls who cheat on their boyfriends, they will have to be in a very shitty relationship to push them that far. I've not known many girls to cheat on their boyfriend unless they got drunk or the relationship was a complete sham.
19-26 (College/Young Adults)
In this age group things become more relaxed. Girls aren't so much looking for love, but for experience. They realize that they have the rest of their life to settle down and want to experience as much of it as possible. In this day and age, women are recommended to "shoot for the moon", since society has become less and less male dominated. Thus, having a boyfriend doesn't hold such strict rules. Many girls have loose relationships that they would have no problem dropping in a heartbeat for something better. Girls also become more relaxed with the idea of sex without expectations. Instead of worrying about committment a fair number of them are more willing to have sex and not desire anything else. Think, the type of girl you SNL. Now offcourse, as with the above group there are some exceptions. Girls who are in love almost surely won't cheat. If the relationship is shitty she might cheat.
That is just the categorizations I've come up with to generalize the women of each age group. I do admit, that in the end its her decision to do what ever it is the situation is heating up to. However, the above thoughts can lend credence to the mindset of your targets.
