She told you she She slept with a guy. She didn't tell you she was in love with him. If you feel she doesn't love you and is showing you in actions that you're not loved then why would you stay with her? On the other hand if she is showing you in actions that she loves you, wants to be with you, is showing you that she values you, then why would you even think about leaving her? Just because she had sex with someone before deciding that you were the one she really wanted is NO reason to get your happy pants in a knot.
“The willingness to accept responsibility for one’s own life is the source from which self-respect springs.” ~Joan Didion