I don稚 think anyone really put themselves into this situation properly.
First assume this guy said something to the girlfriend and she like many told him to F off? He becomes very loud and abusive, yelling using obscenities maybe even looking like he would strike.
The other guy comes out and sees his attitude and aggressive manner, what would a reasonable person do if as he approaches saying get away from my pregnant girlfriend do?
I would have done much worse to him than just push him down as it is reasonable to assume a hot head like this might swing and your child, baby or pregnant girlfriend should be protected.
I honestly don稚 think the guy was very calm in his demeanor while he was saying things to the girlfriend or the man that felt threatened enough to push him down.
So think when someone just has something to say they say it and move on
Somone looking for confrontation raises thier voice, blocks travel paths, approaches at accelerated rates, uses vulgar language and in most cases gets in close to you trying to intimidate you. If you let them get to that point you already lost your defensive stand and when/if they snap or decide to strike you and your girlfriend are now in danger for what comes next.
This was not a civil discussion about illegal parking and as I said there is a lot more to the story than we here know.