It was an NTR that was really Netori because the husband of the wife in the sexless marriage (which is why she was frustrated enough to cheat on him) was really a homosexual (with a paramour of his own) after all and the other guy knew it (his gaydar pinged based on the decoration of their house-seriously) and he told her so, but she refused to believe him (she was wracked with guilt for her own cheating) until she spied on her spouse going on a date with his own lover.
It used to be on Fakku before it became a paysite and now it seems to have vanished from the internet. I can't find it anymore using Google!
I seem to remember it was a chapter from a collection with the title of the series being Young Widow's Story or words to that effect...