Captured women and children

[Tib. Gabinius]

Just to get away from details of cruelity:
please remember not all "loosers" and "catched" were slaved.
Caesar e.g. describes after his first adventures in belgica, that he captured a city and all the people had to do was to leave their weapons.
In fact already there would be some more behind it, but it's an example that not all had to go into slavery.
The end of the story was an ambush to the romans after delivering a part of the weapons, after which the roman slaved all.

So, it can be, but it isn't a have to.
Sometimes, it would also not be a good idea. Remember, to stay nearby Caesar, the people who had to leave Alesia.