SUBREDDIT: r/relationships
TITLE: Help. Not sure what to do.
POST: So I (21M) and my girlfriend (22f) have been together for 3 years. My family does not like her and cut off communication with me because of it. She's always been here for me and I've been there for her. I work and she stays at home. She doesn't really talk to anyone else besides me. I found out she's been talking to a guy from NY. We are in Michigan by the way. When I found out initially,she told me they were just friends. At least that's what I thought. I found out she has a fetlife account and has been messaging this guy for quite a while. I eventually saw a message where he was talking about what he wants to do to her sexually. This really hurt my feelings. I brought this up tp her and she just reassured me she's never leaving and only did this because she's lonely since I'm the only person she talks to and I work 5 days a week. At first, I was really upset. I already know I can't live without her. I'm her only source of income and she has a strained family relationship too. After this, she got to talking about threesomes/group sex. I'm not against the idea, but I'm not sure how I feel. It's been one of my fantasies for group sex, but would be self conscious if I didn't perform well. Not sure what to do. I've tried not thinking about it, but I'm at work now and that's all I'm thinking about. I seriously need some advice. If this is the wrong subreddit, please let me know so I can post to the correct one. Thanks in advance.
TL;DR: 
My girlfriend of 3 years has been talking to this guy online and he has mentioned sexual acts. I'm not against group sex, but have reservations. She only talks to me and the other guy. We are her only friends.