Alison decides to keep the baby and to try and see whether she and Ben can forge a relationship. Ben has nothing else going on--and besides, Alison is hot, so he's game. In furtherance of her goal, Alison asks Ben what he usually expects to do on a second date. He responds that he generally expects oral sex (the actual dialogue is far more explicit). And he doesn't seem to be kidding, since he tells her that's what he told his buddies he thought he'd get out of the evening.
And here we have the problem with Knocked Up. How you react to this movie depends on how you react to this scene. The plot of Knocked Up hinges on Alison finding Ben cute and cuddly, a human teddy bear, lovable despite all his surface flaws. The audience must feel the same way about Ben if the movie is going to work its magic on us.
But on what planet would an irresistibly cute teddy bear basically beg for oral sex from a vulnerable woman who is trying to determine whether said teddy bear, a man she barely knows, could be someone with whom she might be able to raise a child? If that is the planet you live on, or a planet you can imagine visiting, or a planet you think exists, then you might be knocked over by Knocked Up.
It's also interesting to note the long sequence that follows the blowjob incident, in which a frantic-seeming Alison goes on a laborious search for the perfect gynecologist, and eventually settles on an avuncular, stable-seeming older man - that is, precisely the kind of solid masculine presence that's absent from her life. (Her own father, one assumes, is either dead or on the lam somewhere east of Suez, since he never bothers to put in an appearance during his daughter's crisis.) Again, this sequence is played for laughs - and I did laugh - but it isn't necessarily funny. Particularly since the movie could have easily sacrificed a few of the horrified yuks - by making Ben clumsy but not quite so crass, poor but not quite so shiftless, etc. - and still been terrifically hilarious. Which is why Denby's right, I think, when he suggests that the film's devotion to "the dissolution of a male pack, the ending of the juvenile male bond," ultimately goes too far and undercuts the marriage plot. Especially since you need look no further than Apatow's own The Forty-Year Old Virgin for an example of a (similarly socially-conservative) raunchfest that manages this balance more effectively.
Meanwhile, Noah Millman has Apatow's next project lined up ...