Are open/cuckold relationships sustainable?

Throwaway to protect mine and my girlfriends privacy. Apologies for long, rambley post. TL;DR at the bottom.

My girlfriend, whom I adore, is really into the idea of having an open relationship, which I'm OK with for the most part, but can't help feeling a little paranoid. We've been dating for nearly 3 years now, in a loving, semi long distance relationship (we go to different universities but see each other most weekends). We're very open with one another about our sexual desires and our sex life is great, so I don't believe she wants this because I'm not satisfying her. She's always been open about her fantasies of having group sex or general sleeping around, so this isn't a recent development. She also wants me to try sleeping around, or at least getting with other women (in a club, for example), but I don't have much interest in doing so for the time being, which she's fine with.

After about 6 months of seriously considering it, I tentatively agreed to try it out to test the waters. I decided to give her a "free pass", so to speak, where I said she could try it with someone and I wouldn't pass judgement or get annoyed, just to see how we both felt about it afterwards, and last week she got with two different women (she's bi) while on holiday with some friends, and would have liked to take it further but she was unable to due to how the night panned out. She was very upfront about it, telling me exactly what went down while in the taxi home. I'll spare you the details, but after getting over the initial shock I'm okay with it for the most part. I actually sort of enjoy the jealousy I experience whenever I think about, it in a perverse way.

Since that night, she has expressed her desire to try it again with other people provided I'm still on board with the idea, and I've yet to decide either way. My main concern is this nagging sense of paranoia that this will end badly for us. I'm scared my ability to please her will diminish if this continues and/or or she'll fall for another man/woman. She's nothing if not loyal to me, and she promises its purely physical, no strings attached.

I guess before I decide to pursue this any further I wanted to know what reddit thinks. I could never discuss this with any of my friends, so if anyone here has any experience with this sort of thing and could give me advice on how sustainable these arrangements tend to be I'd really appreciate the input.

TL;DR: Girlfriend and I have perfectly happy, healthy relationship. She wants to try opening up the relationship. I'm mostly okay with this but worry for the long term consequences.