In the latter, cheating is more a matter of trust than the act of cheating itself. If you have sex with another person, groundhog day it so that it never happened but you still have memory of it, haven't you still broken the trust between you, even though there is no evidence that anything ever took place.
I think you are missing the point of why cheating is a social crime.
People are supposed to be monogamous. We don't have enormous testicles like a pig or a horse. And rearing children takes us longer than any other creature on earth. Its a commitment that two people make to agree to invest their genetics in each other.
Bearing in mind the huge effort of raising a child (in modern day times it costs a lot of money but it would have been a bunch of hunting and gathering in the past), you are both going to want to make sure it is your child that is being looked after. You don't want her cheating because she could fall pregnant to another guy and you spend the next 18 years paying for it. And vice versa, she doesn't want you splitting resources that could go to her child with women you have made pregnant. It is such an investment that there needs to be that mutual trust and breaking it elicits such strong emotions because of the effort required from both parties.
There are also other implications such as disease. She isn't going to want you to bring back VD and give her a blind child.
This is why its a social no-no. This is why people get furious about it. This is why you feel guilt. Its all hardwired. Those primeval feelings make you part of a successful species.
Now you can throw on a condom and have sex with your own sister these days and it actually isn't going to have the 'repercussions' one could previously expect. But my God you are still going to get some serious grief about it because modern day contraception is 50 years old and our cavemen brains aren't. It is not possible to rationalise past those hardwired emotions. People will feel hurt. The Law is catching up. Adultery is no longer a crime. Incest still is. Religion is catching up, divorce is now ok, contraception isn't. But we'll probably evolve gills before a woman isn't pissed at you for cheating.
So stop hand wringing. If you could get away with it and it really was the perfect crime, that's what a perfect crime is. Its perfect. You gained, no one lost.