My girlfriend told me she wants to try lesbian sex. We're in LDR.

So, first some context:
- I'm 23 and my girlfriend is 20;
- We've been together for 6 months while I was studying abroad, lived together most of that time;
- It wasn't even an option to end the relationship for us since we are in deep love, so we've been doing LDR since I came back to my country 2 months ago;
- We've been skyping 90% of the days since I left and text all the time;

So, she has always had this fantasy of being with a girl, though she told me in some occasions that she thinks it would be awkward a 3-way with a girl since she wouldn't feel comfortable with me having sex with another girl, even if she's there. I have mixed feelings about she doing it. I mean, I guess that every-guy fantasy of witnessing lesbian sex comes into play here. But I'm not 100% sure if I'd be ok with my girlfriend having sex with another girl, specialy if I can't be there.

On the other hand, even though we love each other, we're young and I don't want to prevent her from fulfilling her fantasies. We've talked about all of this, and she told me it's not like she has urges and she really really wants to do it. But she's curious and would like to give it a shot so she asked me how I feel about it. She also told me our relationship is way too important for her, so she would never do anything to hurt me.

Even though it turns me on the idea of having a threesome with my girlfriend and another girl, if it's just my girlfriend and the girl without me, somehow the excitment turns into doubt.

So, I would like your opinion about the following:

Is this selfishness, the fact that a threesome with a girl turns me on but if it's just my girlfriend and another girl I'm not so comfortable with it?

She told me she would never play on the lesbo team, she really is just curious. How often would a girl start developing lesbian desires after a first time with another girl, even if her mindset is just fulfilling a fantasy at the time. In other words, what are the chances of her starting to turn lesbian?

My advice to you is that if she opens up that can of worms you have about 90% chance that you continue to want to do this if she has not already while you were gone if she loves you she would never do a thing unless you were there I have already lost two relationships and a marriage to no trust you can't say no to this and you can't say yes to this you just tell her you do what you have to do and see where this takes you