It would be okay for me to hold hands with and hug someone I "barely know," in some circumstances, despite the fact that I am in a long-term (9-year) relationship. The same would be true for my partner (that's significant - if it's okay for one person in a relationship, but not another, the relationship is unhealthy).
It would depend on the circumstances, though. Why is it happening? Do I feel some kind of instant connection? Do I or this other person have an intense need for comfort at the time? Is my partner not available to comfort me? Is there no one available to comfort the other person?
One of the most important questions: would everything happen exactly the same if my partner were present? That is, if I'm comforting this other person for some reason, or feel an intense connection, would I still feel it and feel comfortable acting on it without guilt in front of my current partner? Or is it "naughty" or illicit?
Anything that changes in front of your partner is a warning sign. If the other person were comforting me because my partner were absent, of course, that's different - but I would expect that he or she would feel comfortable hugging me and holding my hand in front of my partner, too, if that were the case, or at least until my partner took over.
Was the situation sexual? If so, is that kind of thing within the bounds of your relationship agreement? Have you made explicit agreements in your relationship, or are you working on some kind of societal default of "we're monogamous, and that's it" ? I have to say that explicit agreements are far easier to maintain than implicit ones.
No, all girls do not cheat, period. If everyone you have been involved with has verifiably been unfaithful to you, I am very worried about your choices. You are repeating hurtful patterns, rather than learning whatever it is you need to learn so that you can move on, as HappyYoga has said. I'd suggest taking some time to be by yourself and examine your past relationships for clues. How do you pick partners? What do they have in common? How can you avoid the same issues in the future?
No matter what anyone tries to tell you, you are not responsible for anyone else's infidelity, so please don't think I'm implying that you are! If you are acting honorably and your agreements with them are clear, the fact that your past partners have not honored their commitments to you is not your fault.
I'm not sure what you mean about someone claiming to be different from other girls. Every individual is unique. Please don't carry your baggage from old relationships into new ones, but do use what you've learned from them to break old patterns. Don't be bitter. Don't be suspicious of every female because you've been involved with bad people in the past.
Good luck to you.