It is not positively making a false statement, but it reveals a mind in hiding - one knowing it has crossed an agreed upon boundary but not capable of admitting the guilt to the other person in the equation. It can be seen as bad as lying because from a psychological point of view, in not telling the truth straight away you are in the same position of avoiding responsibility of breaking an agreed upon rule by choice.
I actually did the above, owned up to it the next day by my own volition and me and the missus haven't had an issue about it since, nor did we really have one when talking about it! Nor do my female friends think that with truth, the actual breaking of the rule is not that much of an issue.
Both are 'not truth'. But silence is also 'not lying'.
This is more of a 'if a mistake has been made, not learning from it is a bigger mistake' kinda thing for me.