I have a theory that solves everything. Incest warning.
Shan and Smash met while in university. They became a couple over time and then one day they fucked. The condom broke, which led to Shan being pregnant. Plarbman was born nine months later and was adopted by another couple. Shan never told thisgirl or Sindy because they were 10 at the time.
This surprisingly didn't affect Shan and Smash's relationship, since they got married several years later.
Around the age of 30; Plarbman met thisgirl in a circus (no idea about the specifics) and they hit it off even though she was ten years older. History repeated itself, only this time the condom was purposefully broken by Plarbman to chain them together. However, it didn't go as planned, as she dumped him when he proposed to her (the summer camp incident Plarbman mentioned). American Dork was born two months later without thisgirl knowing she was pregnant. Long Tom was thisgirl's babysitter, they got married, they had me, the end.