Scoonie theory...

Scoonie is actually an older version of the title character from the short film, "The Tale of RJ," which was the basis for "The Hard Times of RJ Berger."

"The Tale of RJ" came out in 2009, and Neighbors came out in 2014.

In 2014, Scoonie was around 21-22 years old, which means that in 2009 he'd be 16-17 years old. And RJ in "The Tale of RJ"? He's around... 16-17 years old.

Both Scoone and RJ, aside from being played by Christopher Mintz-Plasse, are known for being well-endowed.

For those who haven't seen "The Tale of RJ" (https://www.youtube.com/watch?v=IrnDYuGPPiU), in involves a perpetually bullied nerd who is revealed to be well-endowed at a school dance. Shortly after, word of his size spreads across school and by the end of the short film he finds himself constantly bedding female schoolmates.

Assuming this takes place in 2009 (when the short came out), this would mean that in the five years since, his sexual exploits would become more and more common.

And after going to college, he'd no doubt reinvent himself from a decreasingly-shy nerd into a sexually-active young man.

To him, joining a frat is part of this reinvention. Ladies love his huge penis and ladies love men in frats, so it'd only make sense. And part of being in a frat also entails being part of a culture of male bonding and experimentation with alcohol and drugs.

To that end, much like Ron Jeremy when he started to let himself go, he felt no need to stay in better shape. No matter what, RJ/Scoonie would always have a big dick.

But... what if he DIDN'T put his entire self-worth into his prodigious manhood and his sexual exploits? Technically we already know that. In "The Hard Times of RJ Berger", which is obviously set in an alternate timeline, RJ doesn't identify as being just an overly-endowed guy. He views women as people and not just sexual conquests either waiting to happen or having already happened. He pines for romance, not just meaningless physical connections. Combined with the fact that at the end of the second (and final) season he accidentally gets his female friend/former "girl who had a crush on him" pregnant, he gets a better view on how having sex is more than just a fleeting experience. But prototype RJ/Scoonie didn't learn this lesson. After "The Tale of RJ" he just kept plowing random women. Because he pretty much WAS his big dick. He WAS random sexual exploits.