The decision left many people scratching their heads, with some saying the alternatives to the word mistress were actually much more offensive.
"Always opt for the traditional 'homewrecker'," journalist Christian Schneider tweeted.
"We recommend you go with 'side piece'," another person joked.
Others said that as adultery was "bad" there "should be a negative stigma" around the word mistress.
But some applauded the move, saying it was only right when there was no equivalent term for the man involved and given the fact many people had open marriages.
"If the three of them are in a consensually non-monogamous (open or polyamorous) relationship, the more typical term would be 'secondary partner'," one person tweeted.
"Don't assume that everyone follows monogamist expectations about sexual exclusivity."
According to Inside Hook the "vocabulary bomb" was first introduced by AP Stylebook back in 2016 as there was no male equivalent.
They suggested instead "phrasing that acknowledges both people in the relationship is preferred: 'The two were romantically (or sexually) involved'".
After criticism on Twitter, AP Stylebook acknowledged it's "problematic that the alternative terms fall short".
"But we felt that was better than having one word for a woman and none for the man, and implying that the woman was solely responsible for the affair," they tweeted.