For example, having a Intrauterine Device placed to prevent pregnancy, for no good reason, is a mortal sin.
Not sure why you insert the phrase “no good reason” here.
You are correct that it is possible for a person to have no awareness that contraception is any kind of sin at all. Further, they may not grasp that how the method works might matter, so they might choose methods by considering feasibility, etc. only. Likely for them NFP is just another method of contraception in that they consider it in a long list of possible ways to achieve the same goal, lined up by most effective percent to least. Such lack of awareness can cause the use of contraception for such folks to fail to be a mortal sin.
I just feel like as Catholics we really have a different set of rules and although I believe they are the right ones…I can’t understand how if they are right objectively for us, how do they not apply to everyone?
The rules on contraception are right for everyone, but some people don’t know the rules, so they are excused from serious blame. However, not knowing a rule/fact doesn’t necessarily protect from every consequence that might result. For example, some emotional consequence of usage may be experienced.
Other rules just apply to Catholics. Only Catholics are required to marry according to Catholic form. Protestants are not so obligated. That is because some laws are fixed by the Church, and are not strictly divine law. The requirement for two witnesses at a marriage is not a divine law. It is not immutable. They could require three or only one if they wished. Thus it is not required of Protestants that they observe Catholic form.
Remember, though, ignorance through deliberately burying your head in the sand so you don’t have to know about it, this is not the kind of ignorance that protects you from guilt.