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.
Close friendships by their very nature tend to involve extensive time talking and hanging out one-on-one.
They tend to involve a deep knowledge of the other person's hopes, desires and personality.
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.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.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.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).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.