This is going to keep happening. It's a non-issue because legal paternity has already been established and that's all that anyone (Except the guy raising someone else's kid with a lying spouse) cares about. There is this common and erroneous belief that humans are in aggregate somehow different from other primates. Casual bastardy has been a thing since before we climbed down from the trees. As long as children are something that 'just happens' some percentage of them are going to be raised unknowingly by men they are not related to.
I posted an earlier comment on this thread that someone saw fit to delete without notifying me in any way. In short, that avoiding scenarios like this is one of the benefits/consolations of remaining child-free. It would be nice if something in the faq explained that this is a possibility, and why. In response to OftenBen: I suppose it is has proven evolutionarily successful to impregnate women with a child and to let her and the unwitting partner raise it without taxing your own resources. It might come to be not that evolutionarily successful at all if/when genetic heritage information becomes widely accessible.