There are many reasons that committed people don’t want to get legally married — from being married a few times before, to just not being comfortable with the concept in the first place.
Whatever your reason, it’s personal, and so none of my business! I don’t need to know why you don’t want to make it legal; I just love the fact that you want to celebrate your commitment to one another.
Just like engagement parties celebrate the fact that you’ve reached a new place in your relationship, why not throw a party for your non-legal commitment? Maybe you’d like to surprise people with the news of your new commitment and throw a surprise ceremony at a party, or maybe you’d prefer to go the whole hog and have a wedding in everything but name!