http://www.slate.com/articles/life/dear_prudence/2011/12/dear_prudence_my_husband_is_leaving_me_for_my_25_year_old_daughter.html?wpisrc=obinsite Turns out it's not his own daughter. He's her second husband and the daughter is from the first marriage. So she's an adult not related by blood.
According to Slate, I'm supposed to find this "morally repugnant".
Sorry Slate can't see it. When I think morally repugnant, things like drone planes paid for with our tax dollars murdering children in Afghanistan come to mind, not straight men who want to fuck something younger and hotter than their wives.