I remember a long time ago reading a book on trust and it said if someone hurts you and they do not offer a sincere apology, you can bet that they will do that very same behavior again. It was a real eye opener for me because at the time I was young and thought you were 'supposed' to give folks another chance.
No one should step over a line, but we're human and we do, but once we do we need to apologize, if we don't then trust has been severely impacted. And, as soon as you offer an 'explanation' or after the apology a 'but' or offer any other justifications, in my opinion it nullifies the apology completely. Saying I'm sorry is absolutely imperative in any intimate relationships for exactly the reason witz mentioned, we ALL make mistakes.............but sincerely apologizing is the first step to regain trust. And, let's face it, we all know when an apology is sincere, you can feel that.