Hey there,
Parents are like that, mine are for sure. I suggest you take them aside and ask to talk to them: say you need to talk without them interrupting, and try and word your argument in such a way that it is not an attack on them, but rather a way to ease their fears. For instance, if they say we don't want you doing anything inappropriate, (take a breath before you answer, and look them in the eyes) you can say that you are growing up and need to have more freedom, however, you are sensible and have clear limits, and will not do anything foolish.
It sounds like your parents are seeing this relationship as getting serious (you've stayed together a lot longer than most couples manage) and so this is making them stress about how far you may have gone. They are your parents and it might be a case of you are one of their children who they really don't want to see hurt, or they are trying to be stricter than they have been previously to prevent things that your siblings may have done from happening again. (I'm not trying to give them excuses - you can address these things easily if you talk to them and having these already thought through will support you saying you are mature)
If you don't want to talk, try writing a letter to the - again try not to sound like you are attacking them in any way this wont help at all. Writing a letter to them may lead to a conversation so be prepared for this.
This situation sounds like it would probably be best solved with a calm talk, so any worries can be dealt with
Hope this helps
Feel free to
pm me if you need