Why does the article's title say this is "counter-intuitive"? If anything, it seems completely intuitive to me; promiscuous people, regardless of sex, are most likely to cheat and not want to stay married.
They were doing this so they could filter out the passwords from chats (i.e. to make it so users can't give out their passwords to other users). Not saying this justifies it, but it's interesting.
I've looked at many of those Tumblr posts; most of them show that the website sends you a welcome email with your password in plain text, which is bad practice, but doesn't prove that the password is stored in plain text in the database.