I actually think that really deep down monogamy doesn't work for a fair chunk of the population. The model of having your one life partner to satisfy all of your sexual needs for eternity has a number of problems:
* We are not always in the mood for sex. Your sex drive can be low for days, weeks or even months and it be perfectly normal. Having the pressure to perform when you're not really in the mood is a horrible feeling. This has a side effects of resentment between the couple.
* Repeating the same sexual experience with the same individual with no changes (e.g. exploring kinks, new positions etc) gets monotonous. No matter how attracted you are to that person, monotonous experiences really impact sexual arousal.
I don't think these problems are new society, but I think there were non-ethical outlets in the past. To the first point for example, in a time gone past the good housewife barely had a say in consent. Prostitution, cheating etc were outlets for the second point.
People feel a lot of shame when talking about ethical non-monogamy, particularly slut shaming women. In reality it's the ultimate level of trust in a relationship. I don't want the pressure of having to satisfy all of my partner's sexual needs for eternity, I trust them to explore other individuals if I'm not in the mood.
To be clear, non-monogamy is also not a silver bullet to a failing marriage, nor is it for everyone. However I really think as a society we should be less judgemental and critical of people who have unconventional relationships and keep an open mind for your own.
edit: typos