You mean to tell me, that Monica didn’t know that the President of the United States was a married man?
No, didn't say that nor imply it. Many many young women get caught up in the seduction of powerful men. She knew she was blowing a married man, but I believe she was naive enough to think that something would come of it (no pun intended).
Maybe she thought she would get a "position" in the WH or maybe Billy would be so enamored that he would leave his wife. Or maybe it was just the thrill of doing the President without regard to consequences. Yes, she was naive - Clinton wasn't. He was just a user.