If I can parrot the wise words that Mr. Drake just mumbled in my ear, it's not so much a pull factor as a push factor. It really is that you have people who are dirt poor. India is doing much better now than it was 20 years ago, even as an economy and a nation, but it still has an enormous population of extremely poor people who have enormous difficulty making ends meet.
In a sense, it's more a push that is driven by the supply factors than the demand. There's always demand for cheap labour and forced sexual services. Unfortunately, that's going to happen. That's prevalent everywhere. However, India's involvement is largely because it has such an enormous pool of really poor people who have very few other places to turn.