I was perusing an old (1805) Spanish moral theology tome online, the Compendio Moral Salmaticense. As Spanish is not my primary language, reading it was at times difficult, but I generally could understand it. One thing I found particularly worrisome was a question (the title was written in a “question and answer” format) asking if casual “alias honest” conversations with women are sinful. There were two responses (R.1 and R.2), the first saying that they are “sometimes not only licit, but laudable,” and the second, much longer, saying that “overly familiar, lengthy, or continuous” conversations are often “gravely sinful” by “reason of danger,” and seems to mention that the “many young people of both sexes in our times” who “presume themselves superior to the flames of Babylon” (not sure about the translation here) are “seduced” and are in a continual sin of “lustfulness” and “scandal.”
I personally do not find having conversations with members of the opposite sex to be an occasion of sin, even if long at times (except perhaps of gossip). Do I need to worry about being an occasion of sin by having a lengthy conversation with a girl? Or must I keep all brief and short? I have a scrupulous tendency, and texts like this are definitely no help for me. Rather they tend to make me feel like despairing.
Without seeing an accurate translation of the book you were reading, I do not know the context in which the instructions were given. My guess is that the instructions would become more clear if context is taken into account. For example, a traditional moral theology text of the early-nineteenth-century might conceivably warn those who are priests or vowed religious from too much familiarity with the opposite sex in order to guard their celibate chastity. It might also warn young unmarried people — whom today we would call teenagers — from interacting too familiarly with the opposite sex. Generally speaking, familiarity is undue intimacy between people who generally should be more circumspect in their behavior toward each other.
If you have difficulty with scrupulosity, I recommend that you consider not poking around in moral theology handbooks unless you have guidance from a spiritual director whose balance and orthodoxy you trust. With the proper guidance, it can be helpful to learn more about moral theology; without it, there is a danger of aggravating scruples. Many such handbooks are written for priests and spiritual directors to guide them in the direction of souls and are not intended for a general audience.