I think that this is an area oft of wisdom that many make into Scriptural mandate where there are none. Even the relationships we have with those of the same sex should be God-honoring and pursued in wisdom. The irony I find that for some who make sweeping rules for all on this matter (particularly the "you can *never* have platonic relationships because of the temptation to sin* I feel ironically fall into the thing they decry of our culture: an oversexualization of people and expectation of oversexualization of our relationships with each other. I believe the same root of our liberal culture is the same root of that extreme, that is to see our identity primarily as sexual, primal beings. While I believe that wisdom, when applied rightly, can in some instances mean an abstention from these relationships because of particular sins and proclivities for individuals, I believe that there is great danger in speaking this as a rule for all. It concerns me the implicit ideas of teaching young men, particularly, that the "natural" way to view other women is as prospective sexual partners, and that they will always be in battle with those urges. Similarly, how horrifying to teach young women that they should *expect* that all men are at all times prone to lust over them. I'll be frank that it is unsettling the implicit things we can teach young people if we treat matters such as this as rules of Scripture when this is an area of much wisdom and prudence more than it is mandate.
How do we think that Paul interacted with women like Priscilla? How did Priscilla interact with Apollo? How did Phoebe contribute to the family of God? How did Lydia open her home to others? These and others we see were married women, single women, widows, older and younger, who all were part of the family of God and must have interacted with men who were not their blood brothers, husbands, and fathers.
I actually find the allusion to the members as brothers and sisters a helpful analogy to how we ought to view one another. How does one react to having a conversation with their brother or sister? Nervously because the fear of attraction? Heaven forbid. A better help may be to consider: How do adult brothers and sisters who have left the home interact with each other? I have brothers I am quite close to, but I am also married. Do I share all of my deepest thoughts, worries and emotions with my brother? Do I monopolize his time and day? Is he the one I turn to every evening about my day? Do I ask him about how to make decisions about what to buy for dinner, how to care for my household? Those are usually conversations with my husband. I don't have to know absolutely everything about my brother and his life details. But do I have conversation with my brothers about how our family is doing, how we could partner in strengthening it, share in successes and sadnesses, seek help or wisdom? Yes. It is no threat to my marriage that I have these conversations with my brother. In fact it is often a needed relationship, as we have areas of strength and weakness that support one another, and we work to the overall well-being of the other members of our family. I am able to balance time with my husband and time with my brother that is prudent. How then can that relationship be similar to non-blood brothers and sisters? I would say that there are ways to balance these relationships in a God-honoring way and there is no hard-fast yes or no rule about whether we can or cannot have these relationships. There are even inevitable male-female relationships that must exist in the church such as Elders to their sheep. Do we then bar the women from seeking the counsel of men, and from their Elders coming to know more about their flock, because fear of entanglement? How does that relationship look if not a brother and sister, then perhaps a father and daughter is another helpful relationship analogy.
One area that I think suffers in the church is the discipleship and mentoring of single men and women. It is so hard to be grafted into the church family at this age, and I think it is actually married couples that have some of the best opportunities to help these members along. I think we ought to have Priscillas and Aquillas ready in the church--men and women who are able to mentor young men and women in the faith. For some, perhaps these relationships do involve working as a husband and wife team, for others that may not be the case. But in the least, I think that structure can help us to see the importance of building relationships across sex lines. It is not sufficient that Aquilla only interact with Apollo, but that Priscilla and Aquilla together come and nurture him in the faith; they likewise work together to build and support the church. Women are, after all, to be the helpers to men. This brings into mind those questions of wisdom: How do single/widowed women act as helpers to the church body? How do they support the deacons and elders? If in prayer, then how might they pray more in knowledge without knowledge of the needs of their brothers? If in deed, then how might they act without the direction and instruction of their brothers? How do we disciple those in the church who have not earthly brothers and sisters, mothers and fathers, to go to? Do we cut them off from the privilege of those relationships which are truer in the body of Christ than in the flesh? There are so many examples that we could bring to mind of situations and reasons that we ought to more carefully consider the relationship of the church body rather than a "no" or "yes" answer to interaction. I say all this just to bring up a host of situations where it is clear that there are not hard-fast rules or commands about the relationship between men and women, BUT that we must rely on Scripture and consider the wisdom of Scripture in regards to how the members of the body interact with one another (of which there are many passages)--whether of same or different sex, or different ages, or different stages/situations in life.
I will not have my marriage in the new heaven and new earth, for I will be consummated to Christ and his body. But I will have my brothers and sisters. I hope to have brothers in eternity who I could say were truly my brothers on earth. Likewise, I hope to see sisters in Christ who were truly my sisters on earth. Both who knew me, loved me, and encouraged me in the faith.