Mixed Gender Friendships
Either to seek or to shun society is a fault in one striving to lead a devout life in the world. To shun society implies indifference and contempt for one's neighbours; and to seek it savours of idleness and uselessness. We are told to love one's neighbour as one's self. In token that we love him, we must not avoid being with him, and the test of loving one's self is to be happy when alone. Think first on thyself, says Saint Bernard, and then on other men. So that, if nothing obliges you to mix in society either at home or abroad, retire within yourself, and hold converse with your own heart. But if friends come to you, or there is fitting cause for you to go forth into society, then, my child, by all means go, and meet your neighbour with a kindly glance and a kindly heart. - "Intro to the Devout Life: Society and Solitude" by Church Doctor and Gentleman St. Francis de Sales
It is a blessed thing to love on earth as we hope to love in Heaven, and to begin that friendship here which is to endure for ever there. Jesus Christ loved John, Lazarus, Martha, and Magdalene with specially tender friendships, as we are told in Holy Scripture. We know that St Paul dearly loved St Mark, St Petronilla, Bishop Stachys (Romans 16:9), Timothy, Thecla, St Claudia and Pope Linus (2 Tim 4:21). St Ambrose loved St Monica because of her many virtues, and that she in return loved him as an Angel of God.
Saint Paul reproaching the derangement of the Gentiles, accuses them of being people without affection, that is to say who had no friendship (Romans 1:31). Make yourself affable to the congregation of the poor, humble your soul to the elderly, and bow your head to a great man (Ecclus 4:7). St. Thomas the Universal Doctor, states that friendship is part of the virtue of justice.1
There are some who will tell you that you should avoid all special affection or friendship, as likely to engross the heart, distract the mind, excite jealousy, and what not. But they are confusing things. Sin, even through acts of silent omission, damages or destroys fraternal communion (CCC 1469). Of the difference between true and false friendship:
Worldly friendship ordinarily produces a grand cluster of honeyed words, a cajolery of small passionate endearments from beauty, grace, and sensual qualities.
Sacred friendship has a simple and frank language, praising the virtue and grace of God, the unique foundation on which it subsists.
Laypeople, through the grace of Jesus Christ, require sacred friendships to ensure and assist each other with the many obstacles that they must overcome in the world (Introduction to the Devout Life: Real Friendship). Better are the wounds of a friend, than the deceitful kisses of an enemy (Proverbs 27:6).
We must have congenial friends as members of the Body of Christ. The eye cannot say to the hand, I have no need of you, nor again the head to the feet, I have no need of you. On the contrary, the members of the body that seem to be weaker are indispensable (1 Cor 12:21). For as in one body we have many members, and all the members do not have the same function, so we, though many, are one body in Christ, and individually members one of another (Rom 12:4-5). Therefore, the highest grace does not lie in being without friendships, but in having no friendships which are not good, holy, and true. "Marriage, when rightly understood, is a very real and holy friendship." (Introduction to the Devout Life: On Friendship - Evil & Frivolous Friendship) To a married woman seeking holiness, this advice was written 400 years ago by a bishop, now known as the Gentleman Saint and a Doctor of the Church.
Introduction to the Devout Life by Church Doctor and Gentleman Saint Francis de Sales
On Friendship: Evil and Frivolous Friendship
Frisky Friendships [mitis foltres] / Frivolous Attachments / Amourettes
Of Real Friendship
Of the Difference between True and False Friendship
Remedies against Evil Friendships
Further Advice concerning Intimacies
Of Society and Solitude
1) The Summa Theologica of St. Thomas Aquinas - Question 114. The friendliness which is called affability
PS. I am a godfather today because of the information above.