Are they just friends?

My girlfriend has friends that are a couple. There has been a lot of sexual activity between her and the couple in the past us. When I first met her and then them later she told me nothing has ever happened between any of them. This had made me uncomfortable. She spends a lot of time at their place even when I tell her how I feel and ask her not to... what is really going on? Is it just that they are now only friends and just hang out or is there more?

Are they just friends?
Post Opinion