I was talking about this today with two friends (both Japanese women in their late 20's). I think the opinion of cheating really depends on generation.

You can assess a relationship in two ways. In America we tend to assess in terms of the present, like "what am I feeling now?" But in the older generation in Japan maybe there's more of a tendency to look at things along a continuum, over a long perspective. The idea is that relationships are a process that occur over a long period of time, so cheating in the short term can be forgiven or ignored because this instant is not the only thing that matters. These are both generalizations, but I think there's a ring of truth there as well.

But I think maybe this attitude is changing, I think most young people today won't put up with shit. Like Yuka, how many omiai dates have you been on ;)? People are marrying more for love now.