Various cultures have multiple wives per culture.  Example of this is under Sharia Law. 

Polygamy - According to the Koran a man can have four permanent wives and as many temporary wives as he wants!

With regards to the laws in the UK, children are being born to cousins, but many are being born disabled, thus costing the tax payer a considerable amount of money within the NHS.  Medical data previously suggested that while British Pakistanis were responsible for 3 per cent of all births, they accounted for 30 per cent of British children born with a genetic illness.

Not only this, this is bringing mass issues with economy of the United Kingdom, which the Tax Payer has to pay for.

Looking from another point of view, 2 children per man throughout the UK should be made an important part of the economics of the UK in today's society.

The UK population grew to an estimated 65.1 million in 2015, the largest ever, an increase of just over half a million people since 2014.

Following the relatively high population growth in the UK population during the “baby boom” of the 1960s, growth slowed during the 1970s. In the late 1980s, the population began to grow again when the 1960s’ baby boomers were having children, “echoing” earlier growth. Recent uplifts in population growth have generally coincided with an increase in the number of countries holding EU membership.

It is projected that the population will grow steadily, passing 70 million people in 2026.

The Muslim Council of Britain state - Source: Census 2011. ONS Table KS209EW.1
Just less than half (47.2%) of the Muslim, population is UK-born. Of the non-UK born
Muslims, the greater proportion is from Asia, and Africa. There is also evidence of the settlement of Muslims from other parts of Europe.

The claim is now that by:

“By the year 2050, in a mere 31 years, Britain will be a majority Muslim nation.”

This needs to be addressed accordingly and strategies implemented to make sure the United Kingdom culture, history and resources are protected and that overpopulation of the Muslim Culture, is not going to get out of control.