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).
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.
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.
First Thessalonians 4:1-8 admonishes us not to wrong or "defraud" our brother or sister by implying a marital level of commitment (through sexual involvement) when it does not exist.
As I've discussed before, a broad (but sound) implication of this passage is that "defrauding" could include inappropriate emotional — as well as physical — intimacy.