It's completely up to you what you want to do, and I guess keep in mind that every girl is different, so your gf might feel differently to myself
In my opinion, I don't think her kissing or doing stuff with girls will make her "like girls more". Some bisexuals prefer men, some prefer women, some like both equally. Personally, I'm more sexually attracted to girls, and I don't feel as though sleeping with a guy will change that. However, that doesn't mean I can't maintain a strong emotional (and sexual) relationship with a guy. Being more attracted to one sex doesn't mean you can't maintain a great relationship with the other sex. The feelings she has for you won't disappear if she decides she feels more attracted to girls.
I'm struggling to put this in a clear way so you understand my point of view, so bear with me xD If you don't let her experiment and further down the line she starts to fancy a girl, ends it with you and starts a new relationship with a girl, then that's the same as her seeing another boy that she likes, ending it with you, and getting with him. Whilst men and women have some stereotypical emotional differences, the only real difference is their genitalia. If you make her happy and she makes you happy, she shouldn't really be falling for anyone else at all in the future. Whilst her curiosity is understandable (I've felt the same) she should still be committed to your relationship and should respect your view on this.
I think you should question why she would like to experiment. Does she feel she isn't getting the sexual attention she needs (because you're in an LDR perhaps)? Is she wanting to know if she is definitely bisexual so she can officially 'label' herself? Or is she just wanting a bit of fun with a girl?
Personally, if I was in a relationship with a boy, I wouldn't feel the need for a sexual relationship with a girl and vice versa. Best of luck. I can see this must be really difficult
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