He slept with someone; what's the big deal? Don't you date and sleep with people too? That's his personal life. Whatever he does, unless something illegal, is none of our business nor should we care about it.
This drama was based on real events. Plus, during this time period, it was common for men to marry for political reasons and have more than one wife. When watching a drama, especially a historical one, you need to keep in mind the time setting and customs of that era. If you don't like the idea of the male lead having more than one wife, maybe you should not watch historical dramas, especially the ones that are based on real events.
Now you're in the stars and six-feet's never felt so far"
You will be missed and forever in our hearts.💜
https://support.viki.com/hc/en-us/articles/360034633713-Request-a-TV-Show-or-Movie