Sorry, but if you decide to have kids, you pay for them yourself. Why should I subsidise your lifestyle choices?
-- sfj1642 on twitter
In a sane world, that would be a perfectly reasonable point of view. Unfortunately, back here in the real world, the fact of the matter is this: those kids will one day be paying your (state) pension, your (state) healthcare and any other state benefits or services you may depend on. The state does not make provision for the future, it pays its historical "future debts" out of current cash flow.
That is why it is so vital to the governments of the future that population in the UK continues to grow, whether through immigration or through population growth. If the population ceased to grow, there would not be enough money left for the government to meet the commitments it made generations ago to people still around today.
And changing this is one of the best arguments for changing from a social democrat model of government to a libertarian one: future costs have to be properly funded. Just look at how many pension time bombs are ticking away because of the statist model today. It's not going to get any better by itself.
It's going to hurt some people, probably. But if we don't do it voluntarily and in a carefully-considered manner -- and soon! -- we could really see pensioners dying in the streets, either during my lifetime or those of my children.