*A safe boy space? That would be the world, wouldn't it?
Given that they are the ones doing the raping, the murdering of partners, the violence against other males, etc, etc.*
Boys don't need a safe space from girls.
The phrase 'safe space' irritates the fucking hell out of me.
Usually mentioned alongside 'all men are rapists' blah blah blah.
Also, WTAF has a group of boys under 10 got to do with "...raping, murdering partners" etc? 
Regardless, I do think that both boys and girls benefit from single sex spaces.
Young girls especially benefit from the separation as they are not at risk of being overshadowed or sidelined by the boys (which unfortunately does happen, due to it still being predominately a man's world).
I would hope though that there is much, much more to these 'girls only' groups than bloody arts & crafts.
So for that reason OP, YABVU.
You are also being massively unreasonable to encourage this idea in your son that:
girls = crafts
Perpetuating a pathetically outdated stereotype.