I think Faye is being really selfish here. Bubbles didn’t say she was or was not her first human lover. Instead, Bubbles pointed out that because of her damaged memories she can’t really know for sure. That’s not a “you aren’t my first” it’s a “I CANT know,” which imo is an oblique reminder that the reason Bubbles has been so present focused with Faye is because big parts of her traumatic past are locked away from her. Fate is taking that reminder of trauma as a “you aren’t special” which is just so... ugh, so Faye.
I don't think Faye is being selfish here, but rather she's trying to process the fact that's a great deal of uncertainty in her relationship with Bubbles. But importantly, there's no case of closure about previous relationships.
Consider this, the one time I almost got engaged, it was to a woman who told me about her previous relationships. The last two had been quite abusive towards her, not physically, but one emotionally messed with her and the other left her in debt. But knowing what she had gone through allowed me to try my best for her and the relationship. While the relationship ended, it had been good for her to be in a healthy relationship. But imagine if she hadn't told me because she had forgot about them - how could have dealt with that unseen shadow of her past? What if I kept putting my foot in the wrong time and place? Or worse, what if one of her abusive exes came back into her life and I had no idea who this guy was?
Faye isn't angry that she isn't Bubbles' first, she's upset at the prospect of some spectre from Bubbles' past coming back into Bubs' life, she's afraid that she might lose Bubbles to another. More importantly, Faye is acknowledging she is acting in an irrational manner and removed herself from the situation before she made things worse.