We all dislike WikiPedia, but their explanation might assist you in coming to terms with this reality (I bolded the important part to assist you):
Sub-replacement fertility do not immediately translate into a population decline because of population momentum: recently high fertility rates produce a disproportionately young population, and younger populations have higher birth rates. This is why some nations with sub-replacement fertility still have a growing population, because a relatively large fraction of their population are still of child-bearing age. But if the fertility trend is sustained (and not compensated by immigration), it results in population ageing and population decline. This is forecast for most of the countries of Europe and East Asia.
Current estimates expect the world's total fertility rate to fall below replacement levels by 2050,[18] although population momentum will continue to increase global population for several generations beyond that. The promise of eventual population decline helps reduce concerns of overpopulation, but many[who?] believe the Earth's carrying capacity has already been exceeded and that even a stable population would not be sustainable.
Some believe that not only this (apparent) economic depression we have entered, but the 'Great Depression' of the 1930's (and beyond?) may be, and may have been, the result of a decline in birthrates overall. Clarence L. Barber, an economist at the University of Manitoba, pointed out how demand for housing in the US, for example, began to decline in 1926, due to a decline in 'household formation' (marriage), due, he believed, to the effects of World War I upon society. In early 1929, US housing demand declined precipitously. And, of course, the stock market crash followed in October of that same year. [19]
Even though the overall world population continues to "grow", it is more at the 'back end' than the 'front end' that this is occurring. That is, more people are kept alive than in the past due to improved nutrition, more refrigeration and better sanitation worldwide, as well as health care advances, from vaccines to antibiotics, and many other advances in medications and in different improvements in health care. Certainly, in advanced nations, few groups would be considered to be "breeding like rabbits". The 'baby boom' (1946-1964) in the US, was likely, if Barber's hunches are correct, more of a return to birthrates closer to historical norms, like those of the first decade of the 20th century (but the 'baby boom' of 1946-1964 were still lower than the 1900-1910 period), with birth dearths both before and since making the so-called "baby boom" appear so big. The pig in the snake wasn't so big. It is more that the periods before and after it were so skinny!
Sub-replacement fertility can also change social relations in a society. Fewer children, combined with lower infant mortality has made the death of children a far greater tragedy in the modern world than it was just fifty years ago. Having many families with only one or two children also reduces greatly the number of siblings, aunts and uncles, making this 'demographic winter' much of the world is in not only 'colder', but also much lonelier. This may be the reason that Europeans, overall, appear more reluctant to send their sons to war, including Russians to Afghanistan and Chechnya, than Americans have been (even though US fertility rates are, in some comparisons, only marginally higher).
Population aging poses an economic cost on societies, as the number of elderly retirees rises in relation to the number of young workers. This has been raised as a political issue in France, Germany, and the United States, where many people have advocated policy changes to encourage higher fertility and immigration rates. In France, payments to couples who have children have increased birthrates.[20]
Despite lower birth rates, the increase in human population is still accelerating - with the time between 6 billion and 7 billion being shorter than the time it took for human population to grow from 5 billion to 6 billion - and projections are that we will grow up to 8 billion DESPITE A DECREASE in birth rates even FASTER than it took for us to get to 7 billion.
58% of the world's population live in countries with ABOVE replacement fertility rates - often dramatically well above replacement rates.
If this trend towards lower birth rates continues at the same rate for the next 40 years (a rather dubious proposition) - the world population in 2050 will start to decline.
That is a long way off and by then we may well reach 9 billion people or higher - and absent better ways to sustain this population - a reduction to 8 billion might be a good thing.
People have more children when children are necessary and desirable. It is simple economics. When having children is needed, humans will rise to the occasion. Until then we live in a world with 7 billion people, rapidly becoming a world with 8 billion people.
Good luck beating the drums on the danger of underpopulation in a world with 9 billion people!
And illegal immigrants are an economic drain on these United States - so sad that your delusion has spread over into that area as well.