Only about 3 percent of mammals are monogamous, and orangutans are not one of these. Gibbons, for example, are a type of ape that mates for life, but they have a different reproductive cycle that orangutans. Orangutans reproduce very slowly, and the females are very selective in the process of mating, so what happens is that the males compete in this process with each other.
In captive orangutans, it is shown that females tend to avoid copulation seeking out desirable males, but that the males tend to forcefully initiate it daily, regardless of the resistance they get from females. Also, the males and females differ in size, with males being bigger than females, and when smaller male orangutans copulate with a female, larger orangutans often take their place forcefully.
In conclusion, you can see that orangutans are not monogamous, but rather promiscuous in their reproductive behavior, when you see a bigger male forcefully initiating copulation with a smaller female, displacing the smaller males.