Personal opinion: people living in this world need to have enough social circles, or you don't know enough about your girlfriend's social circle. It is ok for a girlfriend to have sex with the opposite sex. In a way, this kind of thing is normal. However, maybe your girlfriend crossed the line between friends by dating friends of the opposite sex? Whether you should spend more time with your girlfriend may be because of your neglect, which makes her have a desire to meet other opposite sex. Good communication may be the best way to really solve the contradiction between you. I wish you happiness-