You can't intentionally hit your opponent in the balls. Period.
That's an NBA rule Luka Doncic broke on Sunday night -- whacking Collin Sexton right in the man junk during the Mavs vs. Cavs game ... earning him a swift ejection.
The two were battling for position under the basket in the 3rd quarter when things got physical ... and Luka threw what looked like a hammerfist right into Sexton's no-no zone.
The refs called Doncic for a flagrant 2 -- which means automatic ejection -- and when you check out the replay, you can see why.