Because power is an ability to do what one wants, and a lot of people want sex.
I didn't like the book when I first read it (mostly because I seriously dislike Anna, and the book naturally spends a lot of time being told from her POV), but in retrospect, it's brilliant. Tolstoy has, without a single bit of moralizing, presented one of the best arguments I've seen in literature for being faithful to one's spouse and not engaging in affairs.
In South Africa, men marrying multiple wives is still accepted as long as they are "traditional". https://en.wikipedia.org/wiki/Polygamy_in_South_Africa.
Most Muslim-majority countries currently allow polygamy. https://worldpopulationreview.com/country-rankings/countries...
Now whether it was directly admitted to is a different thing, versus a nudge nudge wink wink thing....cue British aristocracy.
We might all have our personal views on whether it's right or wrong. Or whether it's sinful, etc. But that's orthogonal to reality.
I mean... seriously, look at literature, look at music... Heck, read the Bible? It's been a feature of all of recorded human history, that people will stray. We are human as they say...