From a purely reproductive standpoint males are disposable and females are extremely valuable. That's because theoretically u just need one man to impregnate countless women and grow/replenish a peoples population, while on the other hand you would need many many women for this to occur. It's not just because we are physically and mentally stronger than women that men are sent off to war and made to do dangerous jobs, it's also because on a subconscious level humans understand that the majority of men are disposable and not required for the continuation of ones ethnicity, while women are. Why do you think Somali nomads only sell and ship male camels to Arabia and not female ones?
So when Somali women marry out it's a net loss to the population potential of the Somali ethnicity and a net gain for whichever ethnicity she marries into, as she will be having their children and not ours. When a Somali male marries out it's a net gain for our ethnicity because we are increasing the number of women who can give birth to Somali children, since ethnicity in our culture is traced through patrilineal descent. And even if a Somali man decides not to marry and have children it's still not a net loss for us because his reproductive role will and can be replaced by another Somali male via polygamy or serial marriage.
Because humans understand all of the above on a subconscious level, real men have a visceral dislike for other men marrying their women even if they can't articulate why in precise terms.