Polygamy is not concubinage is not having extramarital affairs
Making a promise to someone of exclusive sexual intimacy and then breaking that promise is recognized near universally as a moral failing. He knew such an affair would hurt not just shareholders financially but people emotionally.
Maybe the guy isn't totally morally fiberless, but I think most people would agree that such behavior is a (non-trivial) moral failing.
It meant a woman agreed to be sexually exclusive with a particular man + various legal entitlements. The particular legal entitlements were what distinguished it from concubinage (which was also a woman agreeing to be sexually exclusive with a particular man).
Now that is not what it means in a modern context, but also in a modern context nearly every developed country has below replacement birth rates, meaning that these countries are dying so any particular claims of moral superiority or fitness of the marital/family structure norms of modern societies can basically be entirely disregarded.