1. Acceptable reasons are determined mutually by the couple themselves (supported by a pope's statement in Humanae Vitae?????)
2. Reason must be of great significance and be good for the marital relationship
3. Reasons will differ between couples based on the strengths and circumstances of that particular couple.
Essentially, scrupulosity and judging are not appropriate attitudes to have. Being convinced that someone is contracepting because they aren't pregnant yet smacks of gossipwhy do they need to be sticking their noses in another marriage? I think that's much more of an appropriate question to ask.
Hope this helped! Pax et bonum!
Exactly. People who are trying to live as faithful Catholics should not have time to worry as to why someone else is not getting pregnant. And to gossip or speculate about it is even worse. Hearing that (supposedly) good Catholics are falling into the speculation of others' private matters is really showing how pride and other pitfalls can come up even for some of those who try to follow the teachings. It kind of smacks of other problems. Any time we look that much at others we may very well be taking our eyes off of God in a major way.