Who pays, the boyfriend of the girlfriend?

I have a question I thought I would ask people who are concerned about financial issues and you are a female. My girlfriend, who I have been with for about 1 and a half years, always thinks I should pay for pretty much everything we do. She is a student in law school and currently does not have a job or internship that is paying her. I work full time, making a decent salary, and also go to grad school but part time. Almost every other week we will go out for dinner and I pay. This is even for when we meet up at a Starbucks for a drink or go to a not so fancy restaurant. I have not really been in a relationship before, so this is all new to me, but my girlfriend was once in a 2 year relationship. There have been times when we would go out to eat with her friends and they would pick some fancy place and I would have to spend more than normal. For example, one time when her cousin took us out they paid for dinner and I paid for the drinks. I thought my girlfriend would have covered such costs, since was her plans with her cousin. Then more recently we went to her best friend's birthday dinner and after splitting the bill I had to pay $100 for the both of us and then at the lounge for drinks I had to pay $50. Thus $150 went in one night. I am pretty frugal and try to save as much as I can. Unlike most of our friends around our age (23 years old), I have my own house, two cars that are paid off (they are nothing fancy like a BMW, but 1 of them is pretty nice), and with a house comes rest of the bills (utility, mortgage, basic cable/internet, etc). As far as gifts/presents we have spent an equal amount overall.

My girlfriend seems to think that I am always supposed to be paying for us when we out. Out of our relationship she has maybe covered the bill for dinner about 3 times, and has on occasion paid for things like drinks at Starbucks. She says all of her friend's boyfriends always pay for themselves and their girlfriend. Also none of her friend's boyfriend's have a house or any other costs, since they still live with their parents. Plus some of her friends if they are out of school do have a job and if they are in school most have a paid internship.

Every 3 months we have this kind of talk and we end up arguing over it. What is your take on the whole perspective? Don't get me wrong, I buy her nice jewelry, take her to nice restaurants, etc. Some people have told me that everything should be split until you guys are engaged/married and then everything is pretty much being shared. Relationships, don't always last and I know that is a risk of having a relationship, but I also don't want to spend my money unnecessarily.