Catholics, how is "family planning" any different from other methods of contraception?

The official Catholic position is based on a philosophical concept called "natural law", which states that there is a proper natural function and goal for everything. For sperm and egg, it is to fuse and create humans. Spermicides and barriers make it impossible for this to ever happen, so it's a violation of "natural law". The rhythm method makes conception difficult but not impossible, so it is technically not a violation because there is no barrier to conception, just a lack of participants. That's the legalistic difference: whether the obstacles are active or pive.