Basic biological reality means birth mothers can be certain they're genetically related to their children (setting aside cases of surrogacy and egg donor in vitro fertilisation). By contrast, fathers can't know for sure without a genetic test. This can give rise to misattributed paternity, a situation where a man unknowingly raises a child that isn't genetically related to him or, if he's tricked into this situation, paternity fraud.
Messy relationship disputes can sometimes lead men to subject their children to genetic testing. The rise of consumer genetic testing (to find out more about your ancestors or your health), however, means that today more men are also finding out by accident.