First, here's what has Brian heated ... it's a tweet from a guy named Robby Starbuck -- a self-proclaimed "Free Thinker" who ran for U.S. Congress in 2022 -- who posted a pic of Megan and the kids, with the caption ... "These are Megan Fox’s sons. We used to live in the same gated community and our kids played at the park."

Starbuck adds, "I saw 2 of them have a full on breakdown saying they were forced by their mom to wear girls clothes as their nanny tried to console them. It’s pure child abuse. Pray for them."

Well, Brian tells TMZ ... "It’s a totally bogus story. There are only a few people in their world that can actually verify wether [sic] or not a story like this is true and I can tell you with absolute certainty it is not. This person trying to claim this is true is a perfect example of someone with selfish motives that does not care about negatively affecting a parent child relationship."

To Brian's point, Starbuck's motives in posting the phony claim do reek of political motivation, because he also said, "We knew California wouldn’t do anything about it because the state celebrates this stuff" -- and casually mentions he and his family moving to Tennessee.

Translation: He hates liberal CA, and hightailed it for greener conservative pastures in TN.

For the record, Megan has said she does not tell their children what to wear, particularly their eldest, Noah. Five years ago she did a few interviews saying, “If a boy loves princesses and a girl loves baseball, that’s not indicative of their sexuality. It’s indicative of their communication and creative expression. We can’t limit children by telling them how they should play.”