Kutcher and Portman play Adam and Emma, two young people making their way in Los Angeles with varying degrees of success: Emma — an overachiever who admits that she’s not particularly emotional or affectionate — is a doctor; Adam — irrepressibly warm and affable, if a bit goofy — works as an assistant on a weekly teen-musical show, though he really wants to be a writer. Adam and Emma met years earlier, as kids at summer camp — the movie opens with that flashback, in which young Adam (played by Dylan Hayes) fires the first of the movie’s sexually explicit salvos when he asks Emma bluntly, “Can I finger you?” [. . .] Adam agrees, though of course we know that since he’s just a big mushbug, he’ll be the one to cave in first. And sure enough, he shows up at Emma’s apartment while she — along with two of her roommates, played by Greta Gerwig and Mindy Kaling — are all having their periods. Not only has he brought them cupcakes, which they descend upon with hormonally charged voracious...