The viral version says Switzerland makes guinea pigs marry. The actual rule is less romantic and more consequential. Swiss animal-welfare law recognizes that specified social species need contact with their own kind, and its housing tables require group arrangements or social contact according to species and context. Federal guidance describes guinea pigs as social rodents that need conspecifics and says they should live in groups of at least two.

The principle is behavioral. Food, water, floor area, hiding places, and veterinary care do not exhaust an animal's needs if the species normally organizes life around others. For guinea pigs, a human or a rabbit does not replace another guinea pig. The law and guidance convert that biology into husbandry requirements, treating the opportunity to interact naturally as part of welfare rather than optional enrichment. Other species have different requirements; golden hamsters, for example, are solitary and should be kept alone.

That specificity is why “it is universally illegal to own one guinea pig” is an unreliable summary. The applicable ordinance table, housing situation, and species matter, and temporary or exceptional circumstances are not captured by a meme. There is no pet wedding license. The more interesting legal idea is that responsible care can include a social environment—and that welfare law can ask what kind of life an animal's behavior is built to live.