It is always recommended to use lube whilst having sex. Lube is fantastic at making sex more pleasurable, as well as making the use of condoms more enjoyable. Your girlfriend may, or may not, produce natural vaginal lubricant…but even if she does, I recommend still using lube (or keeping it on the sidelines ready). All women are different in the amount of lubricant they naturally produce and some women produce lots, some very little. There are no rights and wrongs.
The use of lube is recommended for sex between two men, two women and between a man and a woman. Lube makes sex more fun for everyone!