This is my take on it....
You probably want to hear (or did, before you were "done with it") that her apparent flakiness is due to shyness & being overwhelmed, or something like that. It's easier on the ego to think she's just an indecisive person who likes to take it slow rather than to conclude she just doesn't feel strongly enough about you to commit (because INFPs can be VERY decisive when they actually care).
Well, I will burst your bubble by offering an explanation from inside an INFP's head. I may very well be wrong about her, but this is what would lead me to such behavior (and probably has, I am ashamed to say).
I can be curious about someone/something that I don't actually value in any real capacity beyond it being "interesting", & then I may allow myself to be involved a bit because of it. I just want to see what happens. I may also be flattered by the attention, as any person might be. However, I have no intent to commit or take any serious action, because my feeling is unaffected. This person/thing has failed to hit upon any ideal of mine.
I'm pretty nice & sensitive to others in this regard, and so I will usually (99.9% of the time) stop short of exploring the "possibility" before someone gets hurt.
However, sometimes this means I may explore (or have explored) a romance with someone just because I can, because it's something to do. Then, when I realize a commitment is expected, I bow out. However, if someone keeps giving me "new info" which stirs my curiosity, then I can get pulled back in.
I suspect the sex joke made her aware that this is where such flirtations lead - to a relationship & intimacy with stuff like sex.
She may have instigated joked before, but something like that can trigger the whole picture to suddenly come together. That may have made her stop & think about whether she really liked you that way or that much. If she concluded no, then she knew the right thing was to end it.
This is definitely her fault, and probably due to immaturity & a lack of a full grasp of what she was doing, but I doubt she was intentionally, consciously leading you on to boost her ego. The forefront of her mind was likely going "this is interesting & I want to proceed with it", and so it probably felt like genuine interest to her; but likely, there was also a nagging feeling in the back of her mind that she didn't feel a real connection with you, that this was not what she truly wants in a romance. However, the desire to give the benefit of the doubt, to not discount a vague possibility, to accomodate someone else's feeling, can lead to reopening the exploration.
It's best if you just let her go. In short - she's not that into you & likely never was. INFPs may like to take it slow (rather notorious for it), but I think when we truly like someone romantically, then it's a steady slowness that allows a real feeling to grow, not a wavering back & forth because we're not rooted by a real feeling.