This was not marriage under Christian auspices, although there might be an African ceremony; there were few clerics in equatorial Africa, and the "wives" could not marry since they had not been baptized.
The name is European, and reflects similar relationships of Portuguese men, who were the first explorers of the west African coast.
But it antedated European contact; selling a daughter, if not for cash, then for some economic benefit, including simple peace, was a pre-European practice used to integrate the "other" from a differing African ethnic group.
Powerful West African groups with ties to the slave trade used these marriages to strengthen their alliances with European men by marrying off (selling) their daughters.
[4] This practice was particularly useful in ensuring that the children of European men would benefit economically when their fathers left or died.