These are indeed very interesting questions, ashelen and there has been extensive research into this subject here in DK.
So what I'm going to write applies to Denmark during the Occupation and it will be up to you to decide whether you believe it applies to other occupied countries.
It has been estimated that some 30.000-50.000 women and teenagers had some sort of relationship with either German soldiers or Danish collaborators, that is they had a relationship with an enemy.
Why would you wish to form a relationship with someone who had invaded your country?
And why would you have a relationship with a German or a Danish Nazi or even being anywhere near such a person towards the end of the war?
Well, the majority simply fell in love.
Quite a lot did it out of curiosity. The Germans were interesting, something to sample or for the kick of doing something "forbidden".
Probably just as many did it because the Germans were the victors, they looked smart and at least in the beginning of the war, they were winning - and victors are sexy...
Some did it to rebel against their parents in particular or against the society as a whole - i.e. hanging out with the "bad guys" to spite.
Some were basically losers who sought protection and status by hanging out with those "in power".
A few were prostitutes to some degree. With towards the end some 250.000 Germans soldiers in Denmark there was no lack of men who would pay handsomely for female company.
And a relatively low number were Nazis themselves, so to them it was a natural thing to do.
But why would a young women dare go out with a German towards the end of the war, knowing they would be despised, perhaps even having their family and friends turn their back on them, not to mention running the risk of being seized on the street one day and getting a haircut. That happened frequently! The answer for the vast majority is that they were mindbogglingly naive! "Love conquers everything" - "My Fritz is something special" - "Surely nothing will happen to me" - "I wanna live, tomorrow we might be dead" - "I dunno about all that war-stuff. The news are boring and the war is far away. Is my hair alright?" - Psychologically speaking they chose to live in a bubble.
Some where of course aware of the risk, but chose to go on, albeit keeping a low profile.
And then there were those who got pregnant with a German soldier, who acknowledged his child, that also happened quite frequently, especially during the first three years of the war. So they were already engaged to a German or at least the mother of a German-bastard, so they didn't have much of an option.
There were no doubt the odd rape. It is however not something historians have said much about, for the simple reason that the German soldiers were disciplined and the vast majority had absolutely no desire to "rock the boat", when the alternative is a march battalion to the East Front. My grandfather caused a German officer to be shipped directly to the East Front. He was the head of a train and while checking tickets at first class, he encountered a Danish woman with a second class ticket accompanying a German officer (possibly a colonel). My grandfather, not being particularly pleased with the occupation..., demanded the women moved her behind back to second class. The officer got a fit and drew his pistol, but other officers stepped in and assured my grandfather that the officer would be shipped straight back the East Front and true enough the officer was taken away at the next station. That shook my grandfather, who dispite his dislike for the Germans and despite having a son-in-law gone underground (he was a police officer), was a kind man. Everyone knew about the Eastern Front!
What happened to the "field-mattresses" after the war?
The hard cases, Nazis and informers were tried and imprisoned, the rest got acquitted or never even went to court. But instead the "people's court" dealt with them! An unknown number faced public humiliation, especially after the Liberation. We are IMO talking about at the very least several hundred.
Several thousand were ostracized by family and friends and had to move to another part of the country.
An additional number, again several hundred in my estimation, followed their soldier to Germany.
The vast majority agreed with themselves and their family about never ever mentioning their relationship with a word!
Then there were those who already had a child with a German soldier or who became pregnant. Towards the end of the war and after the Liberation that meant that they went to another part of the country and gave birth there. In many cases the child was adopted away and the families kept quiet about this! In the cases where the mother chose to keep his child, there was a very high risk indeed that people would figure out who the father of the child was (DK was even smaller back then) and that meant inevitable
public stigmatization, not only for the mother but for the child as well!
A few chose to stand up and admit they had such a relationship and even ending up marrying in the German, with the support of the family. Then they would face the public reaction as it came.
There is such a case in my family. The granddaughter of such a Danish-German relationship has married into my family and her grandmother wouldn't talk about it. Not even her mother (who was the child in question) was keen to talk about anything. She's dead now and the granddaughter has managed to locate her German "relatives". But that's another story.
Finally, who were these girls?
A very high proportion was actually from the middle class. They were better educated in general and as such could speak German at a more than rudimentary level. There was less social control with middle class girls, in the sense that they had better opportunities to be mobile and meet Germans. In contrast working class girl, were less mobile, found it more difficult to hide a relationship and was under more strict social control. A working class girl dated a German, chances was that the whole neighborhood knew within days.
I hope that answered your questions.