I guess it depends what you want from these Chennai girls.... Since women make up over 50% of the population in Chennai, it should be pretty easy to find females.
But I'm guessing you're looking for some sort of romantic involvement, right?
Well there's always dating sites like http://in.match.com/ - I would definitely suggest the more well-known or reputable ones, as the rest are just scams.
You could also use Facebook's graph search to find women in Chennai, but there's no guarantee they will accept your friend request.
As for the "p" word (prostitution), it is legal and tolerated in parts of India but also frowned upon and quite a shady thing to get involved in. The Merina Beach area is well-known in Chennai for this type of service.
Interestingly, most prostitutes are very easy to find - and there are reported to be 1500 of them in Chennai alone.
Here is some footage that not only proves that, but also argues that they could do with a safe "red-light district" to operate out of - rather than taking a risk on the streets:
If you are interested in that kind of service, I also discovered this question that will give you more details about the sex workers and their availability in Chennai: //relationships.blurtit.com/1556740/where-to-find-prostitute-in-chennai