I admit we're not talking absolutes here, but almost.
In my experience counseling and writing on this topic, everybody thinks (or at least we'll end up in one of the situations you just talked about.
Unlike most other people of our age and experience, we are (insert favorite answer here) a) really astute students of our own and each other's hearts, b) -clear and talented communicators, c) always honest with each other, even when such honesty entails huge vulnerability for whoever is speaking, d) all of the above." Maybe.
This is especially so in a culture — and a church — that struggles with the widespread sociological trend in its young adults known as "perpetual adolescence." Albert Mohler, Alex and Brett Harris, Candice Watters and other Boundless authors have written about this trend at length.
In fact, the failure of many Christian men to pursue marriage well into their 20s and 30s may be one of the most disturbing results of this trend, but that's another topic for another day.
Let's assume for the sake of argument that your intimate friendship is one of those rare jewels that is devoid of the potential for hurt or confusion. In the past, when both sexual immorality and intimate male-female friendships were much less accepted and less common in society, men and women moved more deliberately toward marriage earlier in life.
By offering a taste of the companionship and interactions that make marriage so satisfying, with none of the accompanying commitments or responsibilities entailed in marriage, intimate friendships discourage the pursuit of the grown-up, God-intended outlet for marital desires — marriage.
Yes, I know, the other person is an adult who is free and responsible to walk away if he or she is so unsatisfied, but like it or not, it tends not to work that way. Especially if it's the woman in this position (as seems to be the case more often than not) she will likely feel that if she pushes for something more than friendship, she may lose the interaction and companionship she currently has.
Still, given her desire for a husband — and perhaps to have man as her husband — the status quo of "just really good friends but nothing more for some odd reason" will leave her unsatisfied, frustrated and confused.
Would want to date someone knowing that he or she had a significant, pre-existing and ongoing emotional bond with another single member of the opposite sex?
If I were a single person desiring marriage, the answers to these questions would matter to me.
Either way, that person is now hanging on to the "friendship" in the hope of getting something more despite the "clear words" from the other person that he or she wants nothing beyond friendship.
To the extent that one person's romantic feelings have been clearly articulated to the other (and were met with an unfavorable response) to continue in some no-man's land of "good friends," is arguably to take selfish advantage of the vulnerable party. What if one person develops romantic feelings in a friendship in which no "clear words" have been spoken, such that the desires of the other person are a mystery?
Why risk harm to your own heart or to that of a brother or sister to have a type of companionship that, outside of marriage, is arguably questionable anyway?