It should be an affair, or a relationship between men and women, because girls are very close names, and only when two people have a good relationship will they be called. Of course, it is also possible that only two people are really good friends. It is also possible. From the address alone, I can't see any problem. Maybe their relationship is familiar, or maybe they are already lovers. Besides, we have to look at their expressions and attitudes when they talk to each other.