After reading things about the show, I don't believe for a hot second Megan didn't know Sarah and Michael were married, that's why. Also, she's still talking to him, which speaks volumes about her attitude and willingness to carry on a relationship with a married man. I think Megan and Sarah are both morons, but Megan is the worst of the two. Sarah has kids by him (as ill advised as she was conceiving them.) She has a reason for needing to stay in contact (arguably.) Megan doesn't.