Just do it
I am no expert in these areas but thought i'd stick my twopenneth in seeing as some of the other peoples reply's were probably asmuch use as a chocolate fire guard.
You seem pretty open minded if your doing that kind of thing to yourself, so i'll be quite frank with my reply.
Try it...try all of it...that's the only way you will know. You may be gay, bi, or just curious, but your not gonna know until you have a go. My friend is gay and says he's always known but admitting it to his friends and himself was whole different story. Does your girlfriend turn you on? Not just in sex but to look at? These are all little clues as to whether you can differentiate from the two sides or whether you want to be in the middle!
If you are to intimidated by coming on to a man or going the whole way with a man then get your girlfriend to use a dildo or a strap on and try that for a start. If it's the whole lack of "male" connection that you need then you need to be looking at finishing with your girlfriend, regardless of gender, cheating is cheating mate and not nice for the other person...unless of course she would be ok with it.
Can you talk to her about it? A lot of men, and I mean straight men, do this and enjoy it (fingers up bum I mean) so it's not unusual or weird in anyway, just experimental and pleasurable because that's were the male G spot is located.
hope this helps 
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