I don't think thus forum can accurately evaluate how considerate or how creepy any letter was without seeing all the details. From what floundering says, I don't think such an evaluation would matter. The facts as presented mean floundering and gf are back on track.
Floundering... I hope you and gf are able to figure out what gf was looking for. If your both mono-wired but fell into a poly situation, then please check for shirt circuits so you aren't shocked in the future.
Though it's moot at this point, let me offer some sage advice...
1) You and any partner's other partner have something in common: you both care about that partner in common. Having something in common is an automatic reason to have affinity with someone else. Just beware the beast if jealousy.
2) No matter how scary any new partner is for an existing partner, that new person had some value that caught the attention of your partner. Sometimes it is better to focus on what about that person attracted your partner as a way to enjoy that variety rather than worry about competition or flaws.
Poly or mono or don't know... Be good to each other. *hug*