I have a couple who got engaged before the future husband went off to WWII and never returned. How can I track such an event on Ancestry.com? (It would seem from the web site the assumption is that if a couple got engaged then they eventually got married, which isn't the case here.)
The data experts and or Ancestry.com experts may have more to contribute on this topic, but I suspect you will need to apply a custom tag (or fact/event).
At least in Family Tree Maker 2010 for Mac, "engagement" is one of the optional tags (fact/event) in the "marriage group."
Certain events/facts/tags have particular genealogical significance. For example birth, marriage, and death. Genealogical software relies on the specific data about these "events" for many of the charts, reports and/or narratives developed. Generically speaking, software programs may have a group of optional events/tags/facts that report about these genealogically significant items.
In your case, I suspect that you are trying to apply a tag/event/fact that is assigned to what is Ancestry.com's "marriage group," and so Ancestry is treating it as a marriage record substitute/equivalent.
To achieve a different outcome, my suggestion is that you add a custom event (say "Engaged"). This will move the tag/event/fact away from selections that are reserved for the genealogically significant "marriage group." (In Ancestry.com, I suspect you will need to apply it separately to both the intended parties.)
There is a separate event for engagement. You can create the event manually and then add a source through the "facts and events" tab.