The study seems to mention that the bullies have children at an earlier age. I’d be willing to guess that the relation between having more children and bullying is purely correlative and has no direct impact. Instead, it seems significantly more likely that impulsiveness drives both bullying behavior and unsafe sex, which then leads to more children.
It seems somewhat odd to me that, instead of addressing possible mechanisms of this correlation, the authors talk about how bullying is an evolutionary trait to pass on genes.