The surplus is actually greater than a first examination of statistics will show - as an excess of young boys is quickly turned into a surplus of females due to differential mortality rates. For your information, my examination of the official statistics for 1994 in the United Kingdom shows that a population of 58.4 million people breaks down into 28.6 million males and 29.8 million females. In other words there were 1.2 million more females than males in the United Kingdom at this time. That is a surplus of 1.53 million females. This introduces competition when men are restrcited to one wife.
Any surplus combined with monogamy has a tendency to increase the size of the effective surplus.
This means that 4.08 per cent of the female population cannot marry if they want to. If they are determined it means they have to find a married man and separate him from his wife. Otherwise, in their competition with other single women they just have to offer a better deal - which may often simply mean sex without marriage. This means that, in an allegedly monogamous society, men do not have to marry to get sex, or even female companionship. They can have prostitutes from the excess of women.