... but man, it takes its time getting there.
Ben is one of the most hopeless boneheaded slackers I have run across in movies (and never, thank the Lord, in real life). Why lovely, competent Alison gives him a second look, much less a tumble, is hard to fathom. I guess this is the disbelief we are meant to suspend. Fair enough.
So Ben and Alison find themselves pregnant. They both try to do the right thing, very clumsily. It does take him an amazingly long time to get to Step One - move out of the Dim Slackers Pothouse and get a real job. Still, he gets there. And it is plausible, I suppose, that she does not tell him what to do, because she does not want to be as controlling a wife as her sister is.
Nonetheless, adulthood is not so scary that it really needs to be resisted so forcefully.
I will not go further in the plot to avoid spoilers. It will not surprise anyone who has seen the trailer that the film's tone is sweet, pro-baby, and ultimately pro-marriage.