I’m a little confused regarding your use of the word evolution. If you mean in a biological sense then you have to think about what the selection pressure is in our society (very different in those areas of the third world where there may be times with very harsh selection pressure).
Most of us breed, some more than others. The majority of individuals get to pass on their genes to future generations. As such, although the relative frequency of genes in the population is almost inevitably changing, the selection pressure does not appear to be strong due to the very low levels of mortality in humans prior to reproduction. Furthermore, our constant mixing of genes between different populations reduces the potential for strong directional selection pressure. Does that make sense.
Anyways, the end result is that you seem to be using the word evolution as if it was a ladder of moral advancement or a ladder of improvement, when in fact that would only be the case if those with the highest and consistent morals (as if we could agree who they were) were more successful at breeding.
I think this will put adultury in perspective. Most species (even those that we think form life long pairs) have what we biologists call extra pair copulations. In other words they cheat. This helps pass on their genes (also because it means their own genes have mixed with a variety of partners which increases the chance that at least some of their offspring are adapted to future environments) and therefore help to ensure their survival in the genome of future populations. We are no different.
The other way to look at it is to consider that just 3–5% max of mammals form monogamous pairs, and I believe all the evidence suggests that even in these situations partners cheat.
In fact, some scientists have comapred the testile size in primates to see if you can use them to predict the mating system. For instance, monogamous gorillas have tiny testicles and control their females through dominance. Chimpanzees have massive testicles for their size and ensure their paternity through passing on massive amounts of sperm to as many females as possible. Humans are in between in testicle size, because although females and males are faithful some of the time, sufficient cheat to have selected for larger testicle size in males.
Sperm speed also points to this. Our sperm speed is closer to Gorillas than Chimps, but still not as slow as Gorillas. This points to monogamy but with some cheating. Which, once again, is exactly what you often find in humans.
The other way to look at the evidence for this is to look at the shape of the human penis. It is built to remove sperm. Hence the perculiar head shape. This is not something needed in a monogamous faithful species.
Also remember that amongst humans some cultures consider multiple wives, or even multiple husbands the normal thing to do(sometimes found in the Himalayas for instance). So what you may think of as immoral (adultery), may or may not be. But it is certainly something that has been favoured biologically for millions of years.
As such, some of the “immoral” acts you discuss, which have to do with reproduction, may in part continue in populations due to the strong evolutionary pressures that form our biology.
BUT this does not mean that it doesn’t hurt, nor is this an “What is, is ought” fallacy. If two people expect honesty and faithfulness, then we can’t use our biology as an excuse, as we are thinking beings who control our behaviour. Also remember that most species have multiple strategies for ensuring offspring survival which can differ between individuals. Some favour not cheating with high parental investment. Some cheat. Some do both. All systems work if they produce offspring.
Hope that helps. SOrry I was in a rush when I wrote this…