If you are adding the 'father', as an example, by putting his ID at the top of the Add parent screen, do NOT select the GO button. Scroll down the page because there will be a checkbox next to the 'mother' or 'his spouse' to state that they were married. If you select the GO, the system will add the 'mother' as a spouse, as if they are married, which you don't want. If that happens, you can just go into one of their profiles, select add/remove spouse, scroll to the bottom of the page and select the one that you want to remove.