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Someone suggested that males and females would react differently to the letter. It's interesting that nowhere in the letter does it say that the LW is a woman. I sort of assumed the ex-couple was two men, given that we heteros are a little less likely to use the "partner" title. Of course, it doesn't change the ethical quandary of the situation!
I tend to think the ex's bad behavior should not be a factor in the LW's decision. The ethical solution should not hinge on the way the relationship ended.