Married or not, infidelity is unforgivable. We can all learn something from the numerous celebrity cheating stories, and it is clear that the Cheryls, Mrs Giggs, and Victoria Beckhams of this world lack self-respect, and are not respected by their husbands.
Marriage is not unconditional as some may delude themselves to believe, it is something that requires effort, trust, and commitment. Just because you say 'I do', wear a wedding ring, and sign a marriage certificate, does not mean that you have to then put up with disgraceful behaviour. You vow to each other to be loyal and commit; both emotionally and morally, and these vows are supposed to be exclusive to your relationship and not involve a third person, otherwise they would have been present at the ceremony to join in!
Walking away from a marriage due to infidelity is not a case of 'giving up at the first hurdle', it is more a case of; not cheating at the first opportunity, and then expecting this 'problem' to be something that is easily worked through. Infidelity is the ultimate betrayal and the betrayed partner has to surrender their happiness and self-respect in order to forgive, and they ultimately commit to a lifetime of paranoia.
Marriage should not be entered into lightly, therefore if you don't want to be wholly committed to just one person, don't get married. If 'marriage is for life; regardless', how far does this 'unconditional' attitude then go? Could you then say that 'marriage is for life; regardless of domestic violence'?
You may choose to forgive, but you will never forget, and this will have an effect on the remainder of your relationship, it is tainted indefinitely. It is much better to be a Sandra Bullock than a Cheryl Cole. Have some self respect.