I've started to see this more and more in WW2 movies and TV shows, such as Band of Brothers and Fury.
I was curious to know how true this was, for both German women and other European women (polish, dutch) etc or was it more of a forced sexual encounter/prostitution for food and goods?
Both, as with many conflicts. Prostitution would've been incredibly common as it is at any time, but genuine romantic relationships and sexual encounters would also have occurred. I personally know a number of veterans who fell in love and eventually married European women during WW2 or are descendants of such marriages. Unfortunately (and again, as with any conflict), rapes occurred as well. There are about 14,000 documented cases of American soldiers raping women in Western Europe (all were court-martialed, it's worth noting, that's how we get the number). They probably wouldn't have had very many sexual relations with Polish women though, considering American forces were never really present in Poland.
There are books on this subject you can read if its of interest of you.
(Behind a paywall) http://www.nytimes.com/2013/05/21/books/rape-by-american-soldiers-in-world-war-ii-france.html?mcubz=3&mtrref=www.google.com&gwh=ACDD8A526DF4BF0A6E08CCAC3F8716CE&gwt=pay