Brian Austin Green Roasts Megan Fox Over 'Achingly Beautiful' MGK Post

Brian Austin Green is still hurting over the Megan Fox breakup, and he's not hiding it -- he's mocking her love for Machine Gun Kelly by waving their kids in her face.

ICYMI, Meg took to IG Wednesday to post a photo of her and MGK, both in towels ... posing for a steamy mirror selfie ... with the gushing caption, "Achingly Beautiful Boy... My heart is yours."

Awww -- good for her, right? Not in Brian's book!

Megan's post seemed to have BAG feeling some sorta way, based on his blistering public response. He posted 4 photos of his sons -- three of them being Megan's -- Bodhi, Journey and Noah, but his caption was the dagger.

He added the same "achingly beautiful" line, but said his heart belongs to his sons. It's an obvious shot with the not-so-subtle message being (we think) ... don't forget you got kids who deserve your love and attention too.

Bitter? Table for at least one ... maybe 2?