Picking up sexy girls in Rosario for casual sex and dating will not be that easy. There is not a lot of great nightlife here and the women are not slutty and easy to get laid with quickly.
However if you are already there or want to visit anyways we might as well share the limited info there is to share. For nightlife there are pick up bars and clubs around, but there is no main bar strip or nightlife district. That makes it a lot harder if you want to do any venue changing to find a bar with more sexy girls in it. The singles nightlife in Palermo in Buenos Aires is much easier to navigate.
Lets start this post on dating and sex with Rosario girls off with the ladies of the night. Then follow that up with the best online dating site in Argentina. Usually when you are traveling to a city to try and hook up with girls you want to rent a condo online as close to the nightlife zone as possible.
Singles bars and nightclubs are spread out throughout the city and you will likely be hopping in taxis to get to them. That makes your logistics a bit tougher, but there are still places worth visiting. It is important to note that the nightlife in this country is very cliquey and social circle oriented. Most people who have experience here will tell you that you need to work your way into a social circle for best results.
You can read about easier places to find sex in South America at that link. You need to be social, get into her circle, and chat with all of them. This is not something that most guys are comfortable with, and frankly it can be pretty hard to pull off. You can also try to meet sexy girls during the day at malls like Alto Rosario Shopping or Portal Rosario.
