I get upset all the time because she s very friendly to other guys to the point where it s flirty. It s in her nature,
First of all - you need to know what makes you happy, what makes her happy, whether or not you trust her, and whether or not a happy balance can be achieved.
That said - I'm willing to bet it her touching / flirtatiousness was a primary quality that drew you to her in the first place.
She sometimes has close guy friends that she ll softly touch from time to time in conversational settings
It may help to realise that People experience social life in 3 primary ways:
For each of us, one of those senses is dominant - sometimes very dominant. It is not just about what we want to do, but about how we experience life - deep down.
Your girlfriend may be one of those for whom touch is the primary sense for experiencing life. If so, she would be much, much less happy never touching anyone again (even with you to touch).
People who's primary experience of life is through touch, are often known as 'touchy feely' people.
So you say you aren't trying to change her, but it's quite possible you are.
Don't get me wrong - there is nothing wrong with you being uncomfortable with it (every person has the right to honest emotions), just as I would say that if it is in her nature (ie. it's her way of expressing friendliness, without venturing into territory where she may stray) - then there is nothing wrong with her touching....it may simply be a compatibility issue, or an accomodation issue, or even compromise (I wouldn't recommend compromise on such a big issue)