When the allies landed in France and worked their way through the countryside the defending Germans abandoned and fled many towns and cities they had once occupied. Soon allied troops would come in and would be deluged with the sights and sounds of celebrating locals happy to be free. It was perfectly natural for many of the local ladies to see these liberators as possessing a lot of Sex appeal. It is no wonder then that under the circumstances they would often literally throw themselves at allied soldiers like they were the biggest sluts on the block.
There were so many cases of local girls practically throwing their pussy at these handsome soldiers that the command could simply do nothing more than let it happen for a time and then try to just keep a lid on it. They knew that allowing their soldiers to get a little singles action was great for the morale and it was good for them to see the happiness restored to people and to know that what they were doing was the right thing. With that in mind they kept discipline as well as they could but also sort of allowed a little looseness to it to some degree. It was a sort of extra bonus they were allowing for. It was basically a way to pretend they had some control over a very uncontrollable celebration, and few would begrudge it anyway.
A little adult action here and there never hurt anyone. Of course many months later there were a lot of children born from these unions and many of them raised these children as their own knowing that their fathers were allied heroes who had come to free them. The sort of Sex in the air after allied soldiers chased the Germans out who had dominated the area for years would have been unstoppable to a greater degree anyway. In the end it was simply natures way of getting things back to normal and restoring peace and prosperity, and a little population boom for good measure.