Assent just means that the default should be agreement with the Magisterium and disagreement is only permissible for a very convincing reason. You have to research the position, find a convincing argument from the Bible, from Scripture, from Saints, etc, before you can dissent to the Magisterium. And even then, I would say you are safest to just assent, because our Church is guaranteed to protect us from grave error. So who would not want to take a guarantee, and instead go with our conscious (which has no such guarantee)?
For an example. Take the death penalty change to the Catechism. This teaching is not an issue of faith and morals, because it says it is now wrong because of enhanced incarceration schemes to effectively detain criminals without resorting to death, and this claim needs to be verified with evidence. We do not believe that the Pope has all knowledge available to him, so that we do not have to even perform studies. Therefore, an atheist professor with knowledge of particular circumstances could be just as likely/ more to be correct.
Denzinger 1158 is definitely an issue of Faith and Morals. I know a poster told you yesterday that it was explained by the circumstances of the time, but this is untrue, the statement is condemning it because of the intentions of the actor. It says “for pleasure only” that means the motive/ intention of the actor. It says nothing about it being wrong because of considerations of poverty, etc.
Let me repeat, there are three sources of morality: circumstances, intentions, and the object. Papal decrees pertaining to the object/ intentions are issues of faith and morals. Therefore, this statement is definitely part of the non-infallible Magisterium. It does require assent, but not full assent!