Thread:TriClear/@comment-26239315-20150606035746/@comment-26239315-20150607060908

From what little info I gathered, it really just seems up in the air at this point and up to pure speculation. My own interpretation surrounding that is that, yes she was having an affair. Leading to her feeling extremely guilty about the entire thing, and when she tries to break off the affair is when Takashi starts getting suspicious, ultimately leading to both of their deaths. Then during the last moments of the dart game, she confesses to the affair, but completely lies about her true feelings for him, therein making Takashi feel a little better about the guilt of potentially killing his child. Along with doing this Machiko not only clears up his conscious to the entire act, but also causes him to be the one reincarnated instead of being sent to the void for both of their deaths.

Again these are just my thoughts about it, we can only go off of what Nona says near the end of the episode and Decim's reaction to her actually telling Takashi these things.