tag:group_sex could not be removed. Check for implications and try again.
someone look into that plz
MMF_threesome implies threesome, which also implies group_sex; that's why you can't remove group_sex without removing the other two tags too.
For future reference, checking the wiki article of a tag will also show you any implications to and from it. This will help you figure out implication relationships in order to remove a problematic tag.
(And yep, I had to check it out too, because I did expect to see mmf_threesome.)