It would reduce possible "family" frictions if the situation where the Unmarried parents of a child , did not show as Husband and Wife. It would be accurate in that situation to list them instead as Male Parent and FemaleParent.
If a "status" of Unmarried was provided as a option (which is currently not an option) and when the option of Unmarried was selected, it could generate the correct format of Male Parent and Female Parent, (instead of the incorrect title of Husband and Wife) list the status of being Unmarried (as opposed to being Married) ... that would be great!
However I think there are already too many "marital status" options. Below are my suggestions for this field of data, the symbol and how such a relationship should display on a printed tree;
Engaged (engagement ring) solid line
Married (wedding ring) solid line
Separated (cracked ring) dashed line
Divorced (broken ring) dashed line
Annulled (certif with X) dashed line
Widowed (as is?) solid line
Unknown (?) dashed line
Plus (for unmarried couples); NEW!!!
Unmarried (heart) solid line (replaces partners)
Ex-Partners (broken heart) dashed line
Other(friends symbol) dashed line (this covers affairs, etc)
The above descriptions of relationship lines on trees only refers to the lines between a couple, not lines to their children.
These changes could be quite easily rolled out and applied to existing databases, relationships currently flagged as "Partners" or "Friends" would become "Unmarried". Users would have to identify "Ex-Partner" relationships as these are currently not catered for and people probably handle them in a variety of different ways.