This is one of those really confusing movies because there are several stories running parallel but they are still connected to each other. Unlike other such films, like The Dead Girl, where you really need to pay attention, in this one, the producers thankfully provide helpful commentary from time to time so that you can read and digest the information.
Briefly, the movie follows the life of several people whose lives are interconnected. Lisa Kudrow was impregnated by her step-brother (Steve Coogan, who turns out be to gay when he grows up) as a teen but rather than having an abortion she gave the baby away for adoption. It turns out that an amateur filmmaker has unearthed evidence of their relationship and wants to film a documentary about their reunion. The step-brother is now in a relationship with a man (David Sutcliffe) and they think that have a child between the four of them with a lesbian couple (Laura Dern and Sarah Clarke). And then there is the third story of Jason Ritter who is still in the closet but when his father (Tom Arnold) finally sees her with Maggie Gyllenhaal he is delighted and tells him to go for her because she reminds him of his wife. Since the son has no interest in women, he ends up falling for her.
Confused already? Watch the movie then. It is a great comedy and an interesting interpretation of modern American life and relationships. And the name Happy Endings, while has its original meaning here, but does not define the movie.