9. Julianne Moore and Amanda Seyfried in Chloe (2009)
Again, we're faced with an unexpected dilemma. Two hot actresses whom we generally enjoy seeing in any sexual context come together for a kiss and...no reaction. We want this to work, we really do. But the May-December relationship between Seyfried and Moore, along with Moore's character's hesitation and jitters remind us too much of a lithe cougar taking down whimpering prey.
Part of the appeal of woman-on-girl sex scenes lies in the illicit naughtiness of the act. Moore's character sure acts like she's doing something wrong, but she's not excited by it. That makes it a lot harder for us to imagine joining in.