I may be wrong, but my understanding that No Jealousy means that if someone spots you making moves on their friend's or their own Romantic Interest/Significant Other/Spouse, it's the other sim who can get in trouble rather than you. So making out with your best friend's wife can harm the relationship between the friend and the wife while your relationship with your best friend is unharmed.
Above Reproach means that a Public Display of Affection is more likely to be ignored and thus have no effect on reputation. But it has no effect on jealousy, so if you get caught you can be the one who is slapped and your relationship with the offended sim is harmed.
Both are a lot more effective if everyone has them. I think it only affects an interaction that is initiated by the one who has it, but if the other sim initiates a romantic interaction your sim's rewards are no longer a factor.