I came home from work early to catch my son getting a blowjob from his girlfriend.

Well, "I came home early to catch my son and his girlfriend" is a little ambiguous. It could mean, "I came home early and caught my son and his girlfriend," or it could mean, "I came home early *for the purpose of* catching my son and his girlfriend." You're meant to think that the phrase is used in the first sense (and in the original version, it is) but in this case it's used in the second sense, and the protagonist is disappointed that he came home early only to find his son and his girlfriend being perfectly innocent. Expectations subverted. (Man, EB White was right about jokes and frogs.)