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.
Yet even with all this deep communication going on, at least one aspect of these friendships inherently involves a mixed message.
No matter how clearly one or both of you have defined what's happening as "just friends," your are constantly saying, "I enjoy being with you and interacting with you in a way that suggests marriage (or at least romantic attraction)." The simple reality (of which most people are aware, whether they admit it or not) is that in the vast majority of these types of relationships, one of the parties involved either began the "friendship" with romantic feelings for the other person or develops them along the way.
They tend to involve the sharing of many aspects of each other's daily lives and routines.
In other words, they tend to involve much of the type of intimacy and companionship involved in — and meant for — marriage.
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?
This brings me to my second argument against intimate one-on-one friendships between brothers and sisters in Christ. Men and women who are not called to long-term singleness and celibacy have a strong desire for companionship with a member of the opposite sex. As I've discussed before, Scripture seems to consider marriage (and children) to be a normal part of the progression toward biblical manhood and womanhood (see, among others, Genesis -28; -24; Matthew -41; Luke -36).
Bottom line: I believe it is difficult and rare — as a practical matter — to honor these principles in the context of a close, intimate friendship between two single Christians of the opposite sex.
(For the verbally precise among you, I think such friendships between non-single Christians are also a bad idea, but that's not what we're talking about here.) Intimate friendships between men and women almost always produce confusion and frustration for at least one of the parties involved.
Romans 13:8-14 calls us to love others, to work for their souls' good rather than looking to please ourselves.
More specifically, verse 10 reminds us that "[l]ove does no harm to its neighbor." Romans 14:1-15:7 offers a discourse on favoring weaker brothers and sisters above ourselves, valuing and encouraging that which is good in the souls of others.
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.