I have to disagree here with you Steven H. - Valerie states it aptly in her comment(s) below: Its biology that drives this enterprise, not legalities or social conventions; our faithful programme writers will catch up eventually.
If WikiTree can create extra buttons for DNA confirmation (read: facilitate serious DNA research), it can also facilitate the correct naming of relationships outside of marriage with extra buttons in the data fields, where it is also visible in the bigger algorithmic tree. See this profile (one of the many now in our project that relies on DNA research): https://www.wikitree.com/wiki/Van_de_Caep-2 (note that the location field in this instance was not used to denote the type of relationship; the relationship still needs to be in a what is up till now only a data field for marriages).
WikiTree is for me a wonderful - stunning - platfom for genealogy and for complimenting the research on other sites, such as the previously named The First Fifty Years Project. (http://www.e-family.co.za/ffy/) I have said it a few times before - the combination of algorithm and dna-research (which I still sadly understand little of but gladly embrace) with scholarly primary validation of genealogical collated data, collaborated [or not] by secondary sources, is (in my humble opinion) the Unique Selling Proposition of WikiTree.
The last few days I have again been wanting to state my gratitude to be part of such an Enterprise; I do appreciate immensely the hard work of maintenance of it by it's leadership and the innovative additions over the years, even when extremely disruptive at times (boiler plates for example). The error data base is one such addition that I find extremely helpful.
To the point - WikiTree also rocks! Up to a point, when it does not serve genealogy because of policy. Then it frustrates - and we are for years now trying to work around those frustrations (such as the use of more than one name in the first name fields because certain cultures do not know the concept of a middle name). And still we love WikiTree, while being direct and honest about it. And patient, as leaders have been so patient with the countless of users of the years.
As Valerie said the program writers will catch up eventually - where there is insight and understanding, innovation must follow. In these times of great strides in technological advances, a "No" is simply not an adequate answer to reasonable requests such as extra buttons on the type sexual relations (which all biological relations are born out of), in the data fields.