I have an Amulet of Mara, but the priest doesn't have the dialog option for marriage.
I have the Hearthfire and Dawnguard DLCs and have adopted one child. Help?
You should still be able to get married even if you already have a child. Both the UESP wiki and the TES wiki mentions instances of your spouse also adopting your children which you had before you were married.
Check if you have completed the requirements for marriage:
You will not be able to marry anyone until you have spoken to Maramal about marriage, even if you have already obtained an amulet from somewhere else. Maramal can be found in the Temple of Mara, but will also spend time in the Bee and Barb.
When you approach an eligible NPC, they may mention the amulet you are wearing. In order for this option to become available, you must complete their objective. Even if a character is flagged as marriageable, the Radiant AI system may not allow you to marry said character even if you have completed their objective, so it's best to have at least one backup candidate.
You can then ask if they are interested. They will respond and ask if you are interested as well. If you say yes, you can then proceed to marriage. In order to complete the event, you will now need to travel back to the Temple of Mara in Riften and arrange the event. Then you need to talk to your spouse (who will be waiting by the doors of the Temple of Mara).