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.
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.
But even if you don't accept that premise, such intimacy is still inadvisable in the sense that it delays and discourages marriage, which Scripture unambiguously calls good and right.
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.
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.