You must get married in the district or county where you obtain the license. So - to answer your question if you get the marriage license in DC you have to have your ceremony in DC. This is true even if MD legalizes.

Your reception however you can have anywhere of course! A lot of couples get married in DC and then have their reception in either MD or Northern VA because it is is CONSIDERABLY less expensive. (Especially N. VA as compared to Baltimore). 

If Maryland legalizes - then MD will recognize your marriage performed in DC. If Maryland does not legalize - MD won't recognize your marriage.