WhatsApp is very useful for private messages between individuals or small groups it is secure , there is no point in using WhatsApp if you want large number of recipients to see the message. Other contacts in your address book cannot see the message, the only way it can be seen by others is if they gain access to your smartphone.
Do remember you are putting your trust in the recipient, forward, copy and paste can be used to send the message to others, which is probably what happened