I'm curious how likely it is that you guys think a girl will be open to cheating?

There's a very hot girl who I see everytime I go to the gym who I'm fairly sure is in a relationship with a guy who occasionally joins her, but I can tell she enjoys me checking her out and I've caught her checking me out and she smiled when I caught sight of her when I first got in today.

Today I also saw another guy chat her up and they had a long conversation which it looked like she was enjoying. Possibly she's single now but I'm also curious if girls are likely to cheat or be open to other guys? Maybe she just likes the attention as she's good looking and always working on her appearance in the gym.

My impression is that people are fairly open to other people at least in the early stages of dating or a relationship though. What do you guys think?