Why west lovers start to have sex with each other immediately after they fall in love?!?

i live in east. When i watch western movies i see something in common in nearly all of them and that is: immediately after two persons start to feel that they love each other, they start to have sex! It means to me that in west, love means sex or at least sex is the manifestation of love! And that is very strange to me because in our culture, always sex is marginal and love is the original, i.e love is the theme, the main point in the relation not sex. In east, It happens a lot that two lovers love each other and they are aware of it but they dont neccessarily have sex. Why it is so (in west)?