Our ethical frameworks aren't too different! I'm a rule utilitarian myself.

I judge the situation as this... You believe lying in this scenario and ensuring the wife never finds out would lead to the most happiness for each party here, which is true.

However, if you sponsored lying for every scenario like this; whether you keep it to just cheating or everything I listed above, then society as a whole would largely be a more untrusting, bitter place. It would be a world where lying is an unquestionable good, provided that telling another party hurts their status quo. A place where you could never depend on someone to show you the kindness of honesty regardless of how much you may want to know about these things. Weighing the good here vs the bad of implementing this train of thought for future scenarios, I have to say honesty is the best policy here.

Encouraging lying as a moral good in situations like this has long-term effects that I wouldn't be satisfied with and I'm reasonably sure a lot of other people wouldn't either; people generally like honest interactions and don't enjoy being lied to.

It's similar to vigilantism. It would oftentimes make complete sense and would be justified- assaulting criminals or killing one to save more could do a lot of good with each action. But if every person relied on vigilantism and this mentality, the world would be filled with vigilantes, which is more dis-favorable to me, due to the unsanctioned assaults and killing of random, oftentimes innocent, segments of the world population. Therefor if someone asks me if they should become one, my answer would be a resounding no.

But... we're not machines, just weighing the math on what actions can do the most good. There are a lot of problems with our frameworks and I can admit that. In day to day life, most people practice moral relativism. But practicing a subjective framework like that for all moral conundrums would be absolutely insane, so my general philosophy is using rule utilitarianism in situations I don't have an easy answer and falling back to gut instincts for the less complex, day-to-day scenarios.