Sometimes it helps to just remember that we're talking about people.
Like most people, Himba men mostly adopt the norms of their culture, and structure the way they think about the world through that lens. For a Himba man, it is normal to love children that were born via another father. Half of them were raised by fathers who were not their own bio-parents! It doesn't represent a threat to his masculinity, because his own father and every man he knows in his social groups has children this way, too. He never expected otherwise. And it doesn't even reduce his number of offspring, because he's having extra-pair children with other women, too.
In the west, on the other hand, the offense in cases of "extra-pair paternity" (to use the article's term) is not usually about genetics. It's a social violation. It offends the husband because his wife has broken an implicit or explicit promise of exclusivity. He can't go have kids himself outside the pair without committing another social violation. It's not normal or accepted in his world, so it makes cultural sense to react negatively to it.
You can ask why Western culture and Himba culture differ in how they think about these things, but then you're not really doing raw EvoPsych - you're doing cultural anthropology.
Like most people, Himba men mostly adopt the norms of their culture, and structure the way they think about the world through that lens. For a Himba man, it is normal to love children that were born via another father. Half of them were raised by fathers who were not their own bio-parents! It doesn't represent a threat to his masculinity, because his own father and every man he knows in his social groups has children this way, too. He never expected otherwise. And it doesn't even reduce his number of offspring, because he's having extra-pair children with other women, too.
In the west, on the other hand, the offense in cases of "extra-pair paternity" (to use the article's term) is not usually about genetics. It's a social violation. It offends the husband because his wife has broken an implicit or explicit promise of exclusivity. He can't go have kids himself outside the pair without committing another social violation. It's not normal or accepted in his world, so it makes cultural sense to react negatively to it.
You can ask why Western culture and Himba culture differ in how they think about these things, but then you're not really doing raw EvoPsych - you're doing cultural anthropology.