She and MC got caught in action, how the hell you think she'd react in a situation like that? Getting caught wasn't part of her plans, and if it was, she'd rather get caught cheating with a random guy, not with her son.
And remember, she did tell her son that her plan IS to divorce Bob, after MC and Sara graduates, so she's determined that the marriage is over, she's determined that her love and sex life will carry on with her son, not with Bob.
How hard is that to understand? She..does...not...give...two...shits..about Bob, she just wants things to progress her way, which ultimately is the best and most sensible way, for everyone