The family trees are already amazing, but yes there is some functionality that could be added. Honestly, Family Tree generation is one of the most cool and unique features of the software, so anything to improve it is great!
Really good point. I have a number of characters who were conceived outside of marriage and, every time, I have to go into the parents' pages to change their relationships.
My family trees are messy with quite a few bastards, options to select something other than spouse would be great
I very much agree with Willow H. R. Harper. It might be a fixable solution to your problem if the developers just renamed the header "spouse" to something less marital. "Partner", possibly?
I'm not against the idea per say, but currently the spouse option is described as including more than just marriage or civil union, even if it is incorrect in terms of definition. Under the familial type box you also have the relationship type you can specify. You can easily describe if they're just lovers, divorcees, and so on, and the text will appear between brackets after the character's name on the sidebar. Which seems to be what you want, but it only appears on the genitor's page, not the offspring's. I typically specify the actual relationship type and in case the "spouse" aspect really bothers me, I just delete that automated character relationship box or edit it as a none-spouse relationship, and both people still remain on the sidebar as the parents. Moreover, I recommend you to display the family tree on both genitors' and offsprings' respective page. From an offspring's perspective, as long as there aren't too many siblings, it will display and indicate who are the full siblings, half siblings and adopted siblings. From a genitor's perspective, you can switch from one "spouse" to another and display the common children. Let me repeat, I'm not against your idea, I do find the familial type system too limited, however, the way your request is written, it seems to partially request already existing features more so than the improvement of features. Which could lead to your request to be rejected. Maybe a follow-up to further clarify your request could be added. (Sorry for the long message)
I would expand this to include "lover" "former lover" "casual tryst" and possible a label for nonconsensual or unknown lovers though I'm not sure what the PC way to label the latter two would be. You could also add more modern or sci-fi parents like "genetic donor" and the like.