two different directions this one could be taken...
movies to turn her on or movies that she will like and will be all fuzzy lovey and warm inside clinging onto you sitting with her
for the second type... for some reason i love john cusack movies. hes just great. ok so high fidelity might not be top choice (although he reminded me of myself in this one) but serendipity, say anything, better off dead (humor angle?), pushing tin even... but the serendipity and say anything choices, go for them.
for turn on movies... depends what shes into... just for looks and appeal of actors/actresses id say orginial sin - and in that vien almost all girls think angelina jolie is a hottie so a movie with her in it can go well- gia, especially if shes into girls
someone mentioned crash, weird movie, but if watching sex thats not classified as having to be found in the back of a movie store behind the 18+ sign then thats a good pick, along with something like eyes wide shut. not subtle by many standards but some girls' taste
some girls really dig when their man will sit through a mushy 'chick flick' with and for them... other really would rather see a good movie