"I am afraid that the pleasantness of an employment does not always evince its propriety."
expresses a sense that there were strong social standards during the period in which Jane Austen composed her novel Sense and Sensibility. Many of the conventions applied to the appropriate relationships between the genders and behaviour for young women. It would have been scandalous for a single woman to be unchaperoned in the company of a young man who was not a close relative.
Good manners and self-control in public were valued; the narrator makes that clear in her approbation of Elinor's sense of duty and condemonation of Marianne:
"it was impossible for her to say what she did not feel, however trivial the occasion; and upon Elinor therefore the whole task of telling lies when politeness required it, always fell." Chapter 21, pg. 104