Here's proof Jake Paul is DEAD SERIOUS about his boxing career ... saying he refuses to hook up with his bombshell model GF, Julia Rose, before his bout against Nate Robinson.

Jake and Nate are on the undercard for the epic Mike Tyson vs. Roy Jones fight this Saturday ... and the Youtuber-turned-boxer is taking a page out of Rocky Balboa's book and abstaining from banging before hitting the ring.

Remember, it was Mickey who famously said "women weaken legs" in the original "Rocky" film ... and superstars like Floyd Mayweather have fully committed to the superstition.

So ... how is Jake's lady taking it??

"She's pissed, man," Jake tells TMZ Sports. "She hates me. She just looks at me like, 'I f**king hate you, bro. Why can't we do this right now?!'"

FYI -- the social media superstar and the Shagmag founder have been together since earlier this year ... and Julia has even helped Paul in the gym while prepping for the Robinson fight.

Aside from sex, Jake says he's also been cutting out food as he works toward making his goal weight of 189 ... so it's safe to say, Saturday can't come soon enough for the dude.