For some people in relationships it's totally fine for their partner to have very close friends of the opposite sex. For other's it makes them uncomfortable. Some of this may have to do with trust, the person's behavior in the past, perceived intentions of the friend, or even seeing how they act together.
My husband's best friend (female) got the news of her dog's cancer days ago. My husband would call her everyday til yesterday when she visited.
I opened the door for her, greeted her then led her to the living room and went into the kitchen to get a class of water after my husband asked me.
I came back and didn't find them in the living room. Turns out they went into the guests room and the door was shut. I heard weeping/sobbing sounds coming from the inside. I knocked then got in and found them embracing each other crying. I stood by the door but my husband paused and told me to give them a moment.