They might claim that this is the way we would act in the wild. Thus they might try to argue that we all ought to use the harem social and mating scheme. Others prefer to point to the way our closest relatives the chimps deal with sex. They use sex to keep group tensions down as well as procreate. Some say that their promiscuity is the natural way we were meant to act.
Dolphins use sex as an integral part of their complex social webs.
Male bonding helps them form teams of three to six dolphins. These teams then work to kidnap females and fend off other groups of rivals. Many like to point to this scheme as most like our own. There is even a case to be made for rape as the natural model for humans. True rape is rare in nature.