LOL.
That's not what I said either.
I realize that INFJs have a tendency to want to bend everyone to their internal vision, but other realities exist. I think INFJs invented the term soul mate, so I understand your unwillingness to accept a different reality.
Of course we can fall in love, "settle down" etc etc....but never in the way INFJs do. We are just wired differently. Very differently.
And I celebrate that difference.
I'm thinking in context of the original post. I'm married to an NT very happily and I don't even know if I would consider him my soul mate, so I'm not sure you can really project that idea onto me or that it's at all relevant to the discussion at hand.
What we've got is a girl (an NF girl) who is in a FWB relationship and thinks she might like it to be more, while the guy says he does not want it to be more. I think it is unwise to stay in such a relationship if she cannot be satisfied with it as it is.
Chances are, if he says he doesn't want a relationship, it isn't going to turn into a relationship regardless of how he behaves or what other things he says that may appear to contradict his original sentiment.
We're talking Ti Se here -- analytical thinking and seeing things as they are (unlike an INTP who thinks analytically but sees all the possibilities and has trouble choosing just one). There is a reason that he doesn't want a relationship. Probably a logical one and that probably isn't just going to magically go away. Of course circumstances could change and his reasoning with it. You just never know.
If she chooses to continue the relationship in the hopes that he may change his mind (and it will turn into a relationship) it could happen, because as you say, ISTPs tend to live in the moment and one day he could wake up and realize he is in love with and wants to settle down with her.
OTOH, there is just as good a chance he could wake up one day and decide he wants to settle down with someone else and she won't hear from again or he might decide he wants to settle down with someone else, but keep her around for sex unless and until he can't get away with it anymore.
Or he could be satisfied with the current status indefinitely or decide he's over it or any number of other possibilities.
If she is okay with that, it's cool, but it's not a comfortable situation for a lot of NFs to be in and, though I have little confidence in my ability to convince, I would like to help her avoid the wasted time and disappointment she's probably going to come to see this as in time. I may not be able to fully get my head around ISTPs, but I have a pretty good grasp of NFs and I have been around long enough to see how this kind of situation generally plays out. It's one of the few advantages of getting old.