Oh yeah - forgot to add - there are two reasons for doing things that I will not accept - one is "I couldn't help myself" and the other is "I was so drunk/high that I didn't know what I was doing". Maybe there are others, but they are equivalent to me.
There are two possibilities for me when someone says that: the first is that they really wanted to do it, but can't think up a good reason why they broke your trust and cheated on you; the second is that they are so lacking in self-control that they can not be relied upon.
Neither for me are a solid foundation for a relationship, I'm afraid.
"Listen, or your tongue will make you deaf." - Native American Proverb