The fact that the person who you are sleeping with is also sleeping with another person or other people does not necessarily mean that he or she does not love you. And the fact that you are the only person who someone is sleeping with does not necessarily mean that he or she loves you.
Being faithful and monogamous is not natural for human beings. It takes work. Deep down we all know that. We have all been tempted to stray at some point or another. Even when it was only a fleeting thought and we didn't act on it. Every time we acknowledge that someone of the opposite sex is "attractive" or "sexy" we are doing nothing other than pointing out that they would be a suitable mate. Not acting on that natural impulse to want to mate with a viable mating partner requires a conscious decision. It's a constant struggle between what your body wants, and what the civilized part of your brain says you should do, in order to avoid the negative consequences of cheating on your spouse and ruining your long-term relationship. That's why affairs, and extra-marital sex, are often referred to as "a moment of weakness.
A healthy world is made of healthy nations. A healthy nation is made of healthy families. And a healthy family can only be raised on the foundation of a monogamous relationship.
Only about 3 percent of animal species are monogamous. A couple of penguins, some otters and a few other oddball critters. To these select few it comes natural to mate for life and never look at another member of the opposite sex. Humans are not part of that little club. Like the other 97% of species, humans are not monogamous by nature. We just pretend that we are.
In view of the frequent occurrence of modern domestic groups that do not consist of, or contain, an exclusive pair-bonded father and mother, I cannot see why anyone should insist that our ancestors were reared in monogamous nuclear families and that pair-bonding is more natural than other arrangements.