The rules vary from group to group. Since you are an international student, you may not have to conform to these unwritten rules, and you may not have to become associated with just one group. You will find students who have very conservative attitudes about sex and other students who do not. Dating is not as common now as it used to be. However, there are students who do still date.
Many girls pay for their own food or entrance to a movie.
Some insist on paying. Some guys feel it is their obligation to pay. It is your decision whether to get sexually involved. Many college students have sexual relationships. However, there are those who do not.