The vast majority of Nigerians I've met have been extremely proud of their heritage and this extends to their romantic life, so a very, very small minority that act otherwise really shouldn't be seen as representative...
..The ones that seem to be thirsting after non-West African women are mostly trying to 'sample' and play around in their youth -- and will almost invariably settle down with their West African beauties once that phase of their life is over.
There is likely a higher percentage of 'Madows' (Nilotes included) that marry out than Somalis, but it's usually with people from the host/mainstream society rather than Horn Africans or other minorities in the West.