Ah dear, the same in our case. We even wrote on our invitations something along the lines of "Hey there, we want to get married. No, we are NOT pregnant."
We didn't think of marrying each other till our 9th year. None of us was into that stuff, we knew that we will stay together and that the other was just right for us. But there were some official reasons that made us change our minds (finances and such) and a possible lifethreatening disease on my side, that made Sward take action. The party was great, we celebrated at home with friends and family. (Never knew that there would be so many to come and congratulate us
) I wasn't into the celebration idea first, I would have been happy to go sign the papers and that's it, but in the end I really liked it the way it went ^.^
Hope you will find the right solution for you OP
Facts: 30, female, bi, v-type relationship with Sward (husband, straight, mono) and Lin (boyfriend, straight, mono), poly-fi and co-primary.