Is it OK to have secrets in my marriage? Are secrets a form of infidelity?

Do you believe that you should tell your spouse or partner everything about yourself and your past? Or are there certain secrets that are “harmless” to your relationship?

Consider the following examples:

1. Patrick and his wife Eve had made a pact to give up smoking together a year ago. Eve managed to stay smoke-free for about three months but after that she started having the odd cigarette in secret and now she is  smoking regularly but is not letting on to Patrick. She feels this is not a big deal because her marriage is strong.

2. 

   Sipho is happily married to Neolitho but is also in regular SMS and e-mail contact with an old girlfriend. He says that his friendship is platonic and that it does not impact negatively on his marriage. He doesn’t tell his wife about it because he says it will only cause misunderstanding.

3. Nancy is happily married to John but she has a secret retail therapy addiction. She regularly spends quite large amounts of money on clothes and accessories and hides her purchases from her husband as she feels that he will not understand or condone her behaviour.
4. Mary has been married for five months and has not yet gathered up the courage to tell her new husband about the abortion she had as a teenager.

Which of these scenarios in your view are acceptable and which would qualify as serious breaches of a relationship?