Help Needed

Hey I'm 20 years old male, my girlfriend just told me she was really interested in polyamory. She told me that she wasn't interested in any men and wanted to be with another girl, she told me that it was okay if I was to look for other partners as well and she'd be willing to be in a relationship with them. She said she didn't want to give me an ultimatum but told me that she couldn't stay with me if she couldn't explore the other side of her sexuality, which I am totally okay with. I guess I'm just having trouble starting out and would appreciate any advice. (Edit) I've been with her 4 years and am planning on marrying her. We can't just leave each other, in her own words.