I might suggest civil rights in general as a historical example.
In the U.S. Antebellum South, the greatest number of people may well have been made happier by the system with slaves. Relatively fewer people are enslaved (about an eighth), and those people generally came from conditions that were not that good to begin with. So their lives may even be improved materially. The Confederate argument is Utilitarian.
But the intrinsic inequality is just wrong, especially when slavery extends across generations. The right action was to free the slaves, even if that threw the South into immediate and quite intractable poverty.
The caste system in India has similarities, but they are not as clear-cut.
Gay marriage is a similar thing. It upsets economic fine-tuning meant to favor reproducing couples without saying so. So its overall effects for everyone may not be positive. And it offends a lot of people in a very visceral way, which makes them less happy. But the arbitrariness of having non-reproducing straight couples treated like reproducing ones, and gay couples treated as not being couples at all, is significantly bad for a small part of the population.
The nuclear weapons trade generalizes out to the whole military-industrial complex with privileged countries supplying weapons for continual third-world wars, of both ours and their own. We like making money of other people's wars. And we like the stimulus that wars of our own give to our economy. And those other people somehow feel better having their wars than not having them, or they would just stop. So everybody may be happier with a huge weapons trade. But in the end, encouraging violence is wasteful of life and resources in a way that is just wrong.