I don't know about you, but I don't care if my wife played hop-scotch with any boys on the school playground before meeting me. But hop-scotch doesn't make new humans, sex does. Even after the invention of effective birth control, sex is still, and always will be, the activity that makes new humans. So this activity carries extra values and implications that other activities don't. The OP suggested that we as a society should move on from old attitudes toward sex and fidelity. But why should our attitudes change regarding sex, when sex is still what it always has been. i.e. the central activity that creates new humans?
I don't know if on the 14 pages of this message board someone has already taken the biological angle to answer the OP's question, but if not, here goes;
When women give birth and raise a child they must pour enormous amounts of physical, emotional, psychological resources into taking care of that little human during young childhood. This is an extended period of time, and the help of a father who will stick with them and help them is incredibly valuable. Finding a man who will do this greatly increases the survival rate of that new child. This is in line with the evolutionary desire to perpetuate your genes in your offspring. So a women increases her evolutionary fitness by partnering with a man who will stick with her and her children. If that man happened to sire offspring before meeting her, it doesn't matter as much to the woman as the question of if he seems like a man who stick with her and help raise THIS child.
On men's part, they want the children that they pour their resources into to actually be their children (this goes along with the evolutionary mechanism to perpetuate his genes through offspring). The man doesn't want to pour resources into another man's offspring's. Here is the critical difference; the man does not have complete certainty that he is the father of said offspring. He can only make a best guess based on the type of woman he mates with. So the man is much more interested in finding a woman who doesn't have a promiscuous history. On the woman's part, there is (normally) no uncertainty about who the father is, so past promiscuity of the father is not as big a concern as how the man she expects the man to act from the present time going forward.
In short, due to difference of certainty of who the father is, men are more concerned with past behavior, but women are more concerned with future behavior. Since pair bonding plays such a big role in successful getting offspring to mating age (i.e single parenting is very hard), this consideration has much more affect in humans when compared to other animals that don't pair bond for as long.
This evolutionary mechanism is encoded into the human race. God made humans with this mechanism at work to best maintain the fitness of the human race. In other words, it is ordained by God to be this way. Of course, God's desire is for all sexual activity to only happen in a marriage, since a family protected by the shield of a committed marriage (man an woman) is the best possible scenario for growing humans. But the evolutionary mechanism I described explains why men and women have unequal CONCERN over past promiscuity.