Ben Affleck Confronts Pushy Fan to Protect J Lo in Venice

Ben Affleck knows boundaries, and when others don't he clearly takes charge, especially in the presence of his GF.

Ben and J Lo were at the Marco Polo International Airport in Venice when a hyper-determined fan zeroed in for a selfie. The fan got in their space and ... you never know someone's intentions when they cross the line, so Ben stepped in for the block.

J Lo's BF pushed the maskless guy away as she moved backward. It's interesting ... their bodyguard took over, wrangled the guy and continued to push him away from Ben and Jen.  So, Ben didn't wait for his hired muscle ... he took care of business himself.

When the dust settled, B&J held hands as they headed for their gate ... but this time they looked down as they walked ... clearly hoping they wouldn't attract fans.

They were in Venice for the film festival but spent a lot of time taking in the sights and getting touchy-feely the way we've all grown to expect.

Ben was there to promote his film, "The Last Duel," which he wrote with Matt Damon.

J Lo wasn't just there for kicks. She modeled for Dolce & Gabbana in a photo shoot.