Many animals form long-term partnerships. Pair bonding is seldom just about sex, even when that is part of the relationship.
Rabbitfish pair up for the long haul but mating has nothing to do with it. Rabbitfish migrate during the new moons of October, November, and December to mate in mass aggregations. Some rabbitfish couples are of the same sex.
Rabbitfish pairs are cooperative alliances, helping them better obtain resources than they could by themselves. Male cheetahs form lifelong alliances to defend territory.
The situation with fish is no different than our own. It’s easier to go through life with a partner and more fun.