Michi couldn't stand Yoichi physically cheating on her despite the fact that he has no feeling for Mishima. Kaede couldn't stand Makoto emotionally cheating on her, saying she would have been OK with Makoto physically cheating since it would just be sexual need. However for Makoto it is "emotional" cheating which is proven to be more serious than physically cheating (like Yoichi) because his heart isn't with Kaede anymore, it is with Michi now.. and that is a lot more cruel than physically cheating I'd say. Physical cheating can be forgiven if it's one time based on spouse, but emotionally cheating cannot be repaired whether forgiven by spouse or not because feeling can't return.
In that sense, Michi and Makoto's "affair" (or romantic relationship) is much more serious than Yoichi and Mishima's affair, from the spouse's stand point. One can be forgiven, since it was just physical. One cannot because it was emotional. That's why I get Kaede when she said she wish it was just physical and she could have forgiven.
