Once Ben got out to the forest he noticed Julia, Matt's sister was out playing with his sister Heather and Matt's black girlfriend Katie. His caveman instinct to mate was high as he was almost completely a caveman at this point. Ben's body was oozing precum by this point and once he caved in to his instincts he let them drive him.
Still retaining some knowledge of how to use the app that made him a caveman and set the devolution to become a virus turning all that come in contact with his bodily fluids to become as hairy and as muscular as he was.
Matt by now was growing tired of waiting for Ben so he went out to find him. When he did he found Ben with another guy under his humongous hands. It was Larry the stalker.
"O-ok man I have $40 in my wallet take it"
Subconsciously Ben sought to punish Larry so he kissed him
"Make you one of us now"
"Like-like a gang or something?" Larry said worried
"Real man Larry" Matt spoke in broken English
Larry at this point started noticing his muscles were growing. The threads of his polo shirt started to shred.
By now Larrys pants were just torn cloth to account for his veiny muscular legs. "All that?" Larry tried to speak in as English as he could remember. Suddenly his face grew long and facial hair longer with his body hair the hairiest. Explosively his balls grew to the size equivalent to Ben and Matts. His penis exploded with precum as it finished growing to the size of a baseball bat.
Now that Larry was a caveman Ben started to grow horny out of lust for Larry while Matt still clung tenaciously to what little humanity he had left. Ben first rammed his huge rod into Larrys ass and pulled out before he let out a huge load all over Larrys backside.
In the midst of the pounding Heather, Katie, and Julia started to investigate the sounds nearby. They were about 2ft from where Larry was being fucked before Ben spotted Julia and in the moment reached for and grabbed her arm.