<human>: Can you condense the text into three sentences or less?

I am probably one of the few men out there that have didn't immediately think "HELL YES" when she brought up wanting to have a threesome, while we were having sex. To be clear, she is Bi and wants to have another female join us. 

To give some context as to why I am concerned: I have been with my GF for about a year now, I am her first BF (not her first time though). About 6 months ago, I made the mistake of drinking a bit too much which ended up with me sexting an old hookup buddy gettting a couple of saucy pics and my GF ended up seeing the entire conversation later on. I should note that I have never physically cheated on her, or set out to intentionally hookup with someone else. That problem was resolved, we had a long discussion and worked it out. She still has moments when she will randomly get upset about it and I always do my best to make her feel better again. On a side note, I am in no way unsatisfied with my relationship, she is one of the best people I know, I consider myself very lucky to have her and try not go a day without telling her how much she means to me. 

Now back to original issue. While we were having sex the other night she started dirty talking about how hot it would be to "share" me with another woman, and how she would "prove she is the best". That last phrase is what made me from "HOLY SHIT" to "holy shit....". She had even suggested that we invite the girl I had previously sexted. I love her so much, but I don't want her thinking she needs to prove her worth to me, because she really is all I need. 

Now the questions is: Should I just go with it? Obviously she is human and capable of having her own fantasies and what not. OR should I talk to her about my concerns? I just don't want to make a big deal out of potentially nothing.
<bot>: GF wants to have a threesome with another girl. My concern is that she is suggesting it because she feels the need to validate herself.