SUBREDDIT: r/relationships
TITLE: My[22M] girlfriend[20F] is having a guy friend spend the weekend at her house
POST: My girlfriend's best friend is a guy. I'll call him Joe. They haven't seen each other in a long time because of distance. About 8 months. They text, call, and skype each other on a regular basis. I have no problem with my girlfriend having a guy best friend, but many aspects of their relationships make me uncomfortable.

I have never met Joe. All I know about him is from what my girlfriend tells me. I know that they have had sex, but those times were mistakes according to my girlfriend. They say I love you to each other. I also saw a mildly inappropriate photo that she sent him. It was a picture of her in only bra and undies. She was showing off that she lost a lot of weight. All of theses things are really hard for me to be cool with. I know that I'm the jealous type, and i know its because of my own insecurities so I haven't told her that I'm bothered by any of this.

Joe is going to stay the weekend at her house. I told her that I'm okay with it, but I'm not. I'm really worried. I trust her, and I don't want to keep her from being with her friend. Am I right to be concerned about any of this? Or am I just being a jealous little boy?
TL;DR: 
Girlfriend is having a guy sleep over her house for the weekend. I'm worried but don't know if I should be. 