I really don't think you will find a person on this earth that is 100% truthfull 100% of the time. Can you really say that, in every situation you've been honest in every case.
The point here is the damage that is caused by lying. I am definitely not right when I lied to her about emailing my ex but that's quite a bit less damaging to someone than having an affair w/ a married man while you date other guys for years and years.