No balls. Science has proven it! 
http://phys.org/news/2014-01-large-testicles-linked-infidelity.html
There is a clear correlation between the size of the testicles of male primates and the proneness to infidelity of females. Learn more about sex, sperm and infidelity at the anniversary exhibition Sexus.
"We can be grateful to parasites, bacteria and viruses for our ability to have sex. Even though sexual reproduction is far less efficient than non-sexual, nearly all living organisms that you can observe in your environment are based on sexual reproduction. The most important function of sex is to reshuffle the genes in each mating. In this way, the immune system develops resistance to parasites, bacteria and viruses," explains Petter Bøckman, Assistant Professor, who has helped produce the new anniversary exhibition Sexus at the Natural History Museum, University of Oslo, Norway.
The correlations between evolution and sperm, infidelity and sex will be a key element of Sexus, which will open in mid-February – and stay open for the entire year – to celebrate the museum's bicentennial.
"Gender is perhaps the most decisive element of our understanding of self in our lives. It is thus important to understand why we are male and female and what this has meant for evolution."
...
In primates, there is a correlation between infidelity and the size of the testicles.
"Here we can determine the degree of fidelity in the female by looking at the size of the male's testicles. The less faithful the female, the larger the male's testicles."
If the male will only fertilize one female and has no competitors, he only needs sufficient sperm to reach the egg. If the female mates on the side, it is smart to have as many cars as possible in the race. Then, the male must have testicles that are as large as possible.
"Bonobos in particular have huge testicles. They mate with everybody. There is an abundant flow of semen. Those who leave the greatest amount of sperm have the largest chance of fathering offspring. In gorilla troops there is only one male. Even though the gorilla has a small harem, he has no need for large testicles – his balls are tiny."
Having testicles that are too large is no advantage, however, since it increases the risk of testicular cancer.
"Animals with short lifespans may have enormously large testicles. In one type of grasshopper the testicles occupy half their body mass."
The testicles are even larger in sea urchins. They spawn directly into the ocean. To increase the chance of fertilizing an egg, the sea urchin is a huge testicle with a little shell around it.
The testicles of humans are one and a half times larger than those of gorillas.
"This testifies with abundant clarity to life in our flock. We can pledge our fidelity until we are blue in the face, but this is evidence that our females are cheating. We are not like chimpanzees, where the female has four or five sexual partners every time she is in heat, but there is always a certain likelihood that the neighbouring male has dropped by."
The testicles are also large in animals that have many females.
"Male lions have huge balls. All the females in the pride must have sex at the same time. When the female lions in the pride are in heat, he must mate with all the females every half hour for three days."
Read more at: http://phys.org/news/2014-01-large-testicles-linked-infidelity.html#jCp
http://phys.org/news/2014-01-large-testicles-linked-infidelity.html
There is a clear correlation between the size of the testicles of male primates and the proneness to infidelity of females. Learn more about sex, sperm and infidelity at the anniversary exhibition Sexus.
"We can be grateful to parasites, bacteria and viruses for our ability to have sex. Even though sexual reproduction is far less efficient than non-sexual, nearly all living organisms that you can observe in your environment are based on sexual reproduction. The most important function of sex is to reshuffle the genes in each mating. In this way, the immune system develops resistance to parasites, bacteria and viruses," explains Petter Bøckman, Assistant Professor, who has helped produce the new anniversary exhibition Sexus at the Natural History Museum, University of Oslo, Norway.
The correlations between evolution and sperm, infidelity and sex will be a key element of Sexus, which will open in mid-February – and stay open for the entire year – to celebrate the museum's bicentennial.
"Gender is perhaps the most decisive element of our understanding of self in our lives. It is thus important to understand why we are male and female and what this has meant for evolution."
...
In primates, there is a correlation between infidelity and the size of the testicles.
"Here we can determine the degree of fidelity in the female by looking at the size of the male's testicles. The less faithful the female, the larger the male's testicles."
If the male will only fertilize one female and has no competitors, he only needs sufficient sperm to reach the egg. If the female mates on the side, it is smart to have as many cars as possible in the race. Then, the male must have testicles that are as large as possible.
"Bonobos in particular have huge testicles. They mate with everybody. There is an abundant flow of semen. Those who leave the greatest amount of sperm have the largest chance of fathering offspring. In gorilla troops there is only one male. Even though the gorilla has a small harem, he has no need for large testicles – his balls are tiny."
Having testicles that are too large is no advantage, however, since it increases the risk of testicular cancer.
"Animals with short lifespans may have enormously large testicles. In one type of grasshopper the testicles occupy half their body mass."
The testicles are even larger in sea urchins. They spawn directly into the ocean. To increase the chance of fertilizing an egg, the sea urchin is a huge testicle with a little shell around it.
The testicles of humans are one and a half times larger than those of gorillas.
"This testifies with abundant clarity to life in our flock. We can pledge our fidelity until we are blue in the face, but this is evidence that our females are cheating. We are not like chimpanzees, where the female has four or five sexual partners every time she is in heat, but there is always a certain likelihood that the neighbouring male has dropped by."
The testicles are also large in animals that have many females.
"Male lions have huge balls. All the females in the pride must have sex at the same time. When the female lions in the pride are in heat, he must mate with all the females every half hour for three days."
Read more at: http://phys.org/news/2014-01-large-testicles-linked-infidelity.html#jCp
