Though there's no law which prohibits unmarried couples to stay together in hotels in India, the hotel industry in general seems to consider it as some kind of a grave crime and a highly illegal matter. Sad, truly. Because, it's not the thought about foreigners, it's only about Indians.
Who is to be blamed for this malpractice? The law enforcement agencies have misinterpreted a government regulation (which says that anyone who checks into a hotel should carry a photo identity proof and submit a photocopy of it at the hotel desk) and made up their own definition and conveyed it to the hoteliers.
Where the law enforcement hasn't done this, the hoteliers made up their own rules. In some cases they have valid reasons to do so.
All 125 crore people of India cannot be good; some are bad and really very bad indeed. There are some common and valid fears in the minds of hoteliers. One terrorist hurls a bomb, one crore innocent people stand in queue and undergo security checking - this is the scenario.
However, there's nothing to be afraid about. In every tourist destination, there are plenty of hoteliers always happy to lodge unmarried couples who have attained the age of eighteen or more. Just submit valid photo ID of both the persons and that's it.
Digha, a seaside tourist destination near Kolkata, sees a huge rush of tourists round the year, a big percentage of them are unmarried couples. If the hoteliers keep turning them back, they know they're losing business.
To give you more hope, the government tourist lodges follow supreme court's directive and they won't prohibit any unmarried couple from staying up in their lodges. I've talked to the officials in the WBTDC booking office and they've confirmed it. I cannot guarantee about such practices in other states, but in West Bengal, things are in right order in this respect.