Sayed sistani says: "All kinds of relations with a non-mahram including joking, expressing mutual love, talking with the intention of deriving pleasure, looking at the body of a girl (except her face and hands up to the wrists) and at her hair or looking at her face with pleasure are Haram (forbidden). In fact, if it is feared that they might fall into a sin, it is forbidden for them to have any kind of relationship with each other."
What is meant by talking with the intention of deriving pleasure. Is it sexual pleasure? For example one might enjoy talking with a na mahram cousin within shari’i boundaries because they have a kind of innate familial affection for them, but they have no sexual intention. What’s the ruling for this? One might also talk about a scientific field with a colleague and enjoy the discussion without lust. Is this prohibited?
What is meant is that the conversation should not stimulate the instinct, even if it is about religious matters, as if they both enjoy each other voice, for example.