Under their Parents, press What are you trying to fix? and choose the option saying the parents are shown as a couple but never were. Not every set of parents was a couple – perfectly fine! The child stays joined to both parents; only the line between the parents changes. For example, your cousin's mum and dad met once and never married: the tree will show them both as his parents, without the couple line.
Where to find it
Person page (Safe edit on) → under Parents, Partners or Children → What are you trying to fix?
Only people with the "Add people" and "Remove" ticks, with Safe edit on.
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