they only abstain during fetile times.
That seems to sum it up pretty well. Except for the mechanics, what’s is there to read up about?
You don’t want kids so you do something to prevent it.
Actually, she's not doing something that would cause it.
That's not quite the same thing, in anybody's morality.
I think you're operating from the tacit assumption that the objective is what determines the morality of an act, but the means by which that objective is achieved also matters.
"Not conceiving right now" is a perfectly moral objective for a married couple in some circumstances. How they achieve that objective is what's at issue.