EXCLUSIVE! WARREN BEATTY’s transgender son has fallen in love – with a man! Inside the secret world of STEPHEN IRA BEATTY!

Stephen Ira Beatty – a 20-year-old college student who was born a girl named Kathlyn – is also becoming an outspoken activist for the transgender community, sources say.

“Stephen is a real firecracker!” an insider told The ENQUIRER.

“He has a quirky sense of humor that he regularly shows off on his Twitter posts.”

Stephen – whose mom is the “Bugsy” star’s wife, actress Annette Bening – attends prestigious Sarah Lawrence College.

His outrageous tweets have featured his opinions on porn, intimate details about his sex life and a confession that he refers to his boyfriend by a nickname that can’t be repeated online.

On Dec. 17, Stephen sent a message to his nearly 400 Twitter followers that said: “It’s the International Day to End Violence Against Sex Workers. The next time someone near you tells a ‘hooker’ joke, tell them to stop.”

On Dec. 22, he announced: “I’m trans and gay. Double the fun!”

As The ENQUIRER reported, it took time for dad Warren to warm to the idea that his little girl is now a man who eventually intends to undergo gender reassignment surgery. But the entire Beatty clan showed their support for Stephen’s lifestyle with a family “coming out” party in Hollywood just a week before Christmas.

AND his famous dad’s acceptance seems to have emboldened Stephen.

“Stephen loves talking about porn and sex in one breath, and then engaging in philosophical political debates in the next,” said the source.

 “He could very well wind up going into politics, which would be ironic, considering that Warren was heavily engaged in political causes when he was younger.

“Maybe Stephen and Warren have MORE in common than either of them ever realized!”