regardless of what the guy thinks, the woman in this case is at fault morally. the kids are innocent.
in the end, if the kids see you as the father, that may be enough for most people. but I can see why some men will walk away.
guys, raise kids on your own risk. these kinds of story are a big part of human history. the fact that sperms from 2 different men will kill each other in the womb goes to show you that it is a fact of life for human existence. H sapiens are not strict monogamous creatures.
Women are also evolved to prefer producing children with fitter and stronger males, but having them raised by more resourceful men (it's really better for their genetic lineage and prospects that way), and men in turn get into a fit of jealous rage over cheating women as a defence against it. Thank god for DNA technology to foil this deception game. Now not only is it a defeat for this guy to have his wife sneak other men's children under his care, but the ultimate insult to injury to be legally obligated to continue to support them financially.
If DNA testing can be used to get a man on the hook for child support, it should also work the other way and get a man off the hook as well.