There are fitful sparks between him (Kutcher) and Portman, but he is unable to sustain viewer interest in his character. She becomes the dominant figure and that throws off No Strings Attached's balance and impacts the all-important chemistry.
Remember the "Seinfeld" episode in which Jerry and Elaine try to become friends with benefits, and set up unsustainable ground rules for their new arrangement? Imagine it rewritten by the Romantic Comeditron 2000 as a profanity-laced schmaltzfest, and you've got this tone-deaf dud.