Are times when the two sexes interact considered sins, or must there be a forbidden act for them to be considered as such?

The relationship between the sexes outside of a valid marriage is forbidden. Conversation between a man and a non-mahram woman is not permissible except out of necessity and with adherence to Sharia guidelines. If it occurs in a manner that is not legitimate, then it is forbidden and entails sin. Marriage is the legitimate path for relationships, and the Prophet, peace and blessings be upon him, said: "Nothing is seen for two who love one another like marriage." If marriage does not occur, then separation should take place.