Caitlyn Jenner That's Not My Boyfriend ... That's My Kid!!

12/24/2015 12:50 AM PST

Caitlyn Jenner: That's Not My Boyfriend ... That's My Kid!!

EXCLUSIVE

Caitlyn Jenner and son Burt got mistaken for boyfriend and girlfriend ... hilarious for sure, but it soon turned ugly.

Caitlyn and 37-year-old Burt Jenner went to lunch Monday in West L.A. when photogs jumped to the conclusion they were hooking up and peppered them with questions. 

Check out the video. You see Caitlyn leave and Burt blocked one of the photogs so he couldn't follow her. When the photog gets out of his car -- presumably to confront Burt -- Caitlyn's son goes on the attack.

We did not include the last part of the video, where the photog starts making vile comments, hurling homophobic slurs and denigrating Caitlyn. It's really awful.