*He also said marriage makes men less violent.
Well, yes, of course marriage makes a violent man less likely to attack people in public when he has a wife and kids to take his abuse out on*
Or, alternatively, it is possible to think about it from a socio-psychological point-of-view and reason :
Men who are married are more often at home than single men, rather than out on the piss, alcohol being a factor in plenty of violent activity.
Men who are married have attained the status of "stable couple", meaning they don't have to (literally) fight for a position in a hierarchy for women's attention (I know it's not smart, but that doesn't mean it doesn't exist).
Men who are married more often have much greater responsibilities - maintaining their contribution to a mortgage, helping in raising children and setting a good example for them - so would reasonably be less prone to using violence and the risk of prison, physical impairment, or death that could result.
But sure, jump to DV being the only reasonable explanation, assuming that you have accepted that a prof of psychology could even be correct such psychological data in the first place...