As far as the Islamic Shari`ah is concerned, interaction with persons from the opposite sex who are not forbidden in marriage is not haram, as long as the intention is pure and all dealings are in line with the teachings of Islam.
However, close friendship with a person from the opposite sex is haram, as it can lead to a haram act, which no one but Allah knows how, where, or when it can happen. This never means the prohibition of pure friendship for the purpose of cooperating with others on the basis of goodness and piety and in compliance with the teachings of Islam.
Zeinab Al-`Alawani, instructor in fiqh and Islamic studies at the Graduate School of Islamic and Social Sciences in Ashburn, Virginia, the US, said, The man-woman relationship is regulated in Islam in a way that secures piety and sincerity of faith. There are certain conditions when it comes to dealing with a person from the opposite sex, such as avoiding khulwa (seclusion of a woman with a non-mahram ), observing the Islamic dress code, speaking properly, restricting the friendship to the frame of work, study, family, etc.
As long as a man and a woman are publicly dealing with each other according to the teachings of Islam, this kind of friendship is pure. However, a married man might feel attracted to a woman other than his wife in a way that urges him to develop a closer relation with her. Such a situation should be avoided, as it can lead to an act that is haram.