I'm not going to bother fleshing this out completely, because the OP is obviously trolling but...
Given the high economic cost of children and the relative ease of preventing pregnancy, people having huge families are disproportionately ultra-conservative folks of one sect or another. If this pattern continues for a few generations, we may see a situation similar to that of ultra-orthodox Jews in NYC, where the traditionalists grow fast enough to throw their weight around and gain radical concessions from the liberal majority. This would be different from social conservative successes of the 70's-90's with their appeals to a "silent majority".
Of course this may not happen, and even if it did, it would be a political divide, not a collapse of liberal society. Who knows when or how that will happen. It could be millennia for all I know
