I was so disappointed in Nels when he cheated on Harriet

I've watched all the little house prairie episodes in chronological order and when the episode came where I saw sweet and caring Nels cheat on Harriet with the Irish lady, I felt disappointed. I know that Harriet was very annoying, insensitive and interfering but I always thought that she loved Nels and her kids. I thought the same with Nels, that no matter how irritating Harriet got, Nels wouldn't cheat on her and have a deep lust for another woman. It just looked like out of character for him and especially on a show like LHOP. The episode was a interesting fun one but it's like I lost faith in Nel after that.