What is probably going on here is sperm competition of some description. The basic idea for any male is to be the man who fathers the most kids, and animals tend to solve this in only a few basic ways. Gorillas do it by mate guarding; there is one male that mates in any one group, and that is all; gorillas are terribly under-endowed even by primate standards with a 1.5" penis, and testes the size of peas.
Chimps go to the other extreme, like this lemur. Enormous testes and a mating system whereby most males in the group that the female permits to have a go, do have a go. The easiest way for males to compete is to maximise the amount of sperm cells they put out, absent of any other mechanism.
Humans, as always, have to be different in mating methods. Human have the largest penis, size for size, of any primate, it is structured differently to most primate penises, and seems to be designed to displace semen from the female genital tract; humans don't produce as much sperm as do chimps, but we produce a great deal more ejaculate with better quality sperm, plus human semen also contains a lot of hormones like Follicle Stimulating Hormone (which stimulates ovulation), so there may be some biochemical warfare going on in humans as well.
What is obvious with mating systems is that old ideas such as birds being very monogamous is complete bollocks. Species like alpine dunnocks also have (in the breeding season) enormous testes, mostly because every male dunnock in any area is mating with any available female in the area, at the same time as trying to stay out of the way of the alpha males in that area. Male dunnocks are thus extremely busy chaps in the egg-laying season, and also very busy afterwards making sure to take food to all the nests where they may have fathered offspring.
I shall leave it to some other intrepid biologist to describe what ducks get up to.