I just called the German register office, last time I called them they said all I need to send them so that I can get a certification of NOT beeing married is my gf's birth certification and a copy of her passport.
So since I have all that stuff today I called them back and found out that the person I was talking to was simply not telling the truth, now I don't know what I have to do, maybe call my local parish or god knows what.
German bureaucracy sucks monekyballs.