Usually (or maybe - always) there're reasons behind any unfaithfulness ...
somehow we have to look ourselves first ,and asking why it can happens..
- do we have enough time for our spouse ?
- do we treat him / her nicely ? with love ?
- do we match each other ?
and thousands questions we ask ourserlves

if after long (or not too long :P) self introspection we find it's not our mistake, than just stand up, take your pride and left the spouse ...

However, the hardest part is can we really look ourselves objectively ??
If we have almost perfect couple, why should we cheat ?

For me, can I forgive ?
If my mistake, I have to !
If not my mistake, why I have to ? :)