I don't know that I accept the idea of "situational" ethics all the time.  Obviously sometimes something is right in one situation and wrong in another, but there is always a deeper ethical principle to be looked for when this happens.   The rule against lying, for example, is sometimes for situational reasons not valid, but a deeper principle, that of not harming someone, is one of the principles that underlies the need for truthfulness (although by no means the only principle -- one could write a book).  So when the truth hurts someone, a falsehood may be appropriate ethical behavior.

When it comes to hiding something from one's spouse -- say an indiscretion -- if it is not going to become a pattern it may be that the lie is the most ethical course, albeit fraught with tar traps.  Both the spouse and the relationship are less hurt and better off long run with the lie.  Problem is it is easy to begin to justify lies with such reasoning and before long one loses track of basic truths.  This leads, guaranteed, to a major train wreck.