If you want to delete a romantic relationship, you'll first have to click on one of the partners you want to delete. An info box will appear on the left side of your screen with information like birth dates, surnames at birth etc. (if you have entered them), and below these information are a few buttons ("Add parents", "Add brother/sister" etc.). The last button is titled "More Actions", and if you click on it, it offers you first the option to set the person's parents, and then "Change partners".
After selecting "Change partners", you'll see all the (current and former) partners this person has had. If your person doesn't have any kids with the partner you want to remove, you'll get the option to "Set [Partner's name] to ex-partner", "Set [partner's name] to extra partner" and "Remove partner [Partner's name]". Now you can choose if you want the two people who shouldn't have a romantic relationship anymore to be ex-partners or delete the relationship altogether (if there had never been a romantic relationship in real life).
However, there are two restrictions to this: 1) If the two people have a child together, you can't delete the relationship, you can only mark the parents as being exes instead of current partners. 2) If one of the partners wouldn't be connected to any other member of the family tree if you deleted the relationship, you can't delete said relationship. You can, however, just delete one of the partners.
