Are Any Animals Truly Monogamous?

Monogamy is rare in the animal kingdom, with only 3% of mammals and 95% of birds practicing it. However, infidelity is common, as seen in superb fairy wrens, where only 25% of offspring are from the social father. This behavior is biologically advantageous, allowing genetic diversity and increased reproductive success. Dimorphism in animals often relates to mating strategies, with non-monogamous species showing pronounced differences. Humans exhibit physical differences, but it's unclear if they indicate a harem-like past or a monogamous yet adulterous society. Perfect fidelity is rare, exemplified by diplozoan paradoxim flatworms.

How does the mating behavior of male gorillas differ from that of male gibbons?

What is unique about the mating behavior of diplozoan paradoxim flatworms?