I think you should evaluate if you really want to continue relationship with this girlfriend, because there is very small chance that she will change with time. Your friend is bisexual and these contacts mean that heterosexually she doesn’t get complete satisfaction that’s why she periodically needs these homosexual contacts which she is carrying out in life. With time she might tend to one or other side.
Creating a serious relationship you should evaluate carefully these facts and whether you accept such sidesteps of you woman. Of course such a behavior is classified as cheating. There’s a big chance that your girlfriend is having such sidesteps not only with same sex people, but with opposite sex too since she doesn’t have any prejudices regarding that. So you should evaluate it yourself, is such a relationship acceptable for you or no and maybe the easiest decision would be to change a girlfriend.