I don't think it's a weapon. You put frustrated young men in an area, who wouldn't otherwise get dates. You give them guns, and they control the area completely with impunity, and they will have sex with women by agreement or by force.
I don't think any soldier ever said, oh well, I don't want to rape this good looking young lady, but since I need to use this weapon of war, I guess I must. This however has been said of a howitzer, so it's not a weapon.
It's the sex drive with no laws to curtail it. If it's a weapon it's the only one that discriminates against old and unattractive women.
Some use pretty cars to get sex, others use the church, some use their oratory skills, some use roofies, others use guns. At the end, everyone is trying to get laid. These guys figured a huge shortcut. And trust me that's all they are doing down there. Not taking territory anymore, raiding towns, discarding the men, and taking the women. And according to reports, not all women are taken. The old and unattractive according to them, aren't. Figure why?
Remember Boko Haram? took 200 girls. Figure why?