I can imagine that for some people, allowing somebody else to use your bed (sleep with your mutual partner in your bed - the one you and that partner usually share) could be a way of controlling the other relationship. (You always have the power to retract the permission.) And for some people, sleeping in somebody else' bed would make you feel at a disadvantage (in their power / "owing them one" - though maybe not that defined or even conscious of it). Maybe lovinhimloviner's partner's GF feels the latter?
lovinhimloviner, at least you changed the sheets! I used to stay at a 3-flat (3-apartment) [young] anarchists' living space whenever I visited the city. I was told to take whatever bed was available, nobody cared about my using their sheets*, and if the bed's usual occupant happened to change plans and come home after I was in their bed, they'd just toddle off and find somebody else' vacant bed.
* I know, I know: I could have taken my own sheets or a sleeping bag, but I wasn't bothered that much either (for the one night). I could always shower before and after... But if I thought that an absence of 2 weeks from my home meant that potentially 14 different people had slept in my bed / sheets, I'd feel a bit queasy.