Hello d&a and do not despair!!

Many couples do exactly what you suggested above, which is to legally marry with a small or civil ceremony (the courthouse basically) in order to start the process for sponsorship. Immigration is very familiar with these circumstances with international marriages as not everyone can manage financially or logistically (nor does everyone want to do) the whole big wedding and honeymoon thing.
The main thing will be documented proof of your relationship together and I am including below a link to a post that I just wrote yesterday so you can read it. In that post is some basic info. and another link regarding how to put together an immigration package but do not fret if yours is smaller or has fewer photos or is not exactly like the one shown.
Link
As you continue to read some of the forum member's experiences and ask more questions and get more answers you will be able to map out a plan that works for you and your boyfriend. The good thing is that being in your position, now knowing that you will be submitting a sponsorship application, you can better prepare to ensure you have photos of the engagement, maybe a party with friends and family to celebrate etc.
Keep track (copies) of all bills, receipts, money transfers, any joint financial transactions, credit card bills, boarding passes, trip itineraries, e-mail communications, facebook, Skype etc. communications. The irony of all of this is that in most relationships, the couple does not have to document or prove that they are a couple. So some of it may feel a bit odd, but in this case even though it might not be natural or feel genuine to do this, immigration needs to see people show evidence to demonstrate that they are a "genuine" couple.
For many of us on this forum, we entered into relationships that are/were long distance (not necessarily thinking of marriage right out of the gate); one of the partners has to do all of the travelling as the other is denied visitor visas (my husband and me

) and in our case we did not even have a camera during the first 3 years of our relationship! Once we decided that we were on the marriage track and we would need to do a sponsorship I had to go back and dig through a lot of things to find some evidence of the early days of our relationship. From that point forward I was like a detective...lol......always saying "gotta keep that for immigration, gotta take a photo of that for immigration" it sounded like "immigration" was my love

!!
So read and research a bit more and understand you are not alone and you will be supported and encouraged by most of the members here. Blessings.