There is this girl I have seen 4 times over a month but I haven't had sex with her yet, she is giving me some resistance. The other day we were catching up and I had to lie to avoid telling her about this other girl I spent a night with. I am wondering what if I just tell her that I slept with this other girl. Do you think that could mess up my chances?

EDIT: My current objective is not setting the ground for an open relationship, it's just to get to sleep with this girl.