Do not present an engagement as a qualifying marriage. The official joining-family description is for an eligible spouse and unmarried children under 21. Marital status is a fact with a date, and the date is what the question asks about. State the position as it stands on the day the form is answered, then record what would have to change, and when, for a different answer to become true.
Use the actual relationship in the application
Answer the official questions accurately and obtain assessment of any legal-recognition issue. A shared address or wedding plan does not itself establish the spouse relationship or guarantee inclusion. Where a wedding is planned, decide before filing which of two structures the couple is actually using: one principal with an eligible spouse joining, or two applications each standing on its own.
That choice has consequences arriving later, so make it deliberately rather than discovering it. The published guidance recommends including intended eligible joining relatives at the outset, which puts the decision at the start of the process rather than at the ceremony. Ask what the file would look like if the marriage happened after filing, and what it would look like if the marriage did not happen at all.
A structure that works on only one of those futures is not yet a plan; it is a preference waiting to be tested.