Every Family Tree site and software is Very UNMODERNIZED, you just have to live with the "Add Spouse" button, there are family tree softwares such as family tree legend, legacy, and family tree builder that allow you to select the two as just friends or for legacy, you can select "this couple never marry". They do pretty good in their reports not to state they were married, though there are certain places they will still say the two were married.
Until they accept the fact that people actually have children outside of marriage and start to add features such as "Add Mother/Father of Child" or something that doesn't imply spouse, you just have to pretend just for them.
This is what I tried, I simply attached the Father to the child and the Mother to the child without attaching the Father and Mother to one another as "Spouse". The problem I encountered, was when you go to either the Mother or father's personal page or predigree chart, it list the child under "Unkonwn Father/Mother".
I didn't like it so I just attached the person as spouse, ancestry and other family tree softwares will have to just get with the program, and I wish they would real soon.