The reason the men are supposed to fight and women are supposed to stay back in a supportive role has a lot to do with how much effort a man and woman relatively have to put into new children. A guy can put in less than a week of effort, a woman will be occupied with the new child for at least nine months, and then will need to recover from the strain.
The labour cost of raising the kid can be more evenly divided, but in that case there's still, historically, a greater burden imposed on women.
As children are a massive resource investment that pay off tremendously for the tribe there's a strong incentive to keep the people most involved with raising them away from combat if possible, to keep the children themselves from involving themselves, to keep them calm and quiet and thus most likely from being found in case the battle goes poorly. Meanwhile the people who are not involved with the tribe's future and/or ready and equipped to deal with threats and are threats themselves to those intending harm can deal with any hostiles.
The labour cost of raising the kid can be more evenly divided, but in that case there's still, historically, a greater burden imposed on women.
As children are a massive resource investment that pay off tremendously for the tribe there's a strong incentive to keep the people most involved with raising them away from combat if possible, to keep the children themselves from involving themselves, to keep them calm and quiet and thus most likely from being found in case the battle goes poorly. Meanwhile the people who are not involved with the tribe's future and/or ready and equipped to deal with threats and are threats themselves to those intending harm can deal with any hostiles.