I always read it as being about defying the law (backed by divine enforcement):

His father commanded him to "... fulfill [his] duty to her as a brother-in-law to raise up offspring for [his] brother", which implies that this was considered a legitimate obligation. His transgression, then, was that he pulled out "to keep from providing offspring for his brother", actively refusing to fulfill that obligation. In that reading, it's a tale about obeying the orders and customs of your elders.

Of course, these don't have to be exclusive: "These norms exist for a reason, so you should damn well obey them."