FTM has setup a structure that it thinks is helping its users - maybe even from customer requests: of identifying step-children as such in the system.
This has unintended consequences and I suggest to NEVER put children as step-children to any parent and, in fact, never ATTACH step children to a second parent. An exception to the latter situation might be to reflect ADOPTIVE parents, but just be sure the BLOOD parents are shown as the PREFERRED PARENTS.
The problem is that the system will consider these children, and their step-parents, as parents for filters, reports and charts, and in other ways that will lead to misleading results.
For example, ALL of the descendants of a step-child will end up in Descendant Chart or Descendant Genealogy Report - with no indication that the person is only a step-descendant in that report (except for the very first one - if the option to "identify relationship type" box is checked in the report right panel.
To undo what you have done, you don't really need to undo the step relationship - what you need to do is to select the child and DETACH them from the step parent via the Detach screen.