My hubby and I each have several friends of our own same sex... And have had overnight visitors in the past of some of those friends. Sometimes I'm there, sometimes I'm not, and vice versa.
Its never really bothered me one way or the other... My hubby is a charming, sweet man, who flirts and picks and plays but in the end, I know who he's gonna be sleeping with.
I could just be odd, but I'm not the jealous or possessive type... And once I trust someone (which goes without saying here or I wouldn't have married him), I trust them implicitly. He's mine, and I'm his and nothing or no one is going to change that.
You keep saying that you *know* nothing is going to happen, so why let it bother you? Who cares what anyone else thinks? If anyone says something to you, you will always have the upper hand by the very fact that it wasn't a secret... You knew that the friend was staying overnight, and you trusted your boyfriend enough not to be insecure about it.