A friend of mine which had a similar aversion had himself diagnosed and treated for minor OCD and I believe he handles the situation better now. Without saying you have OCD or that you should follow therapy, you should consider that, in the situation you describe:
- It's very tempting for someone to touch your phone if you are showing something on it.
- People try to do so in order to help you.
- It's a socially accepted behavior to let professionals touch some of your belongings, think of people checking your ID card for example, or even medical staff touching you whenever required.
Despite the above, you consider it inappropriate: one will question where that feeling comes from and why it is causing you so such negative feelings when someone touches your phone. Obviously, the situation would be simpler if you complied to the norm.
Imagine a situation where it would have been helpful for someone to touch your phone, and you deny that request. What can that person think about your refusal? Try to put yourself in the shoes of your interlocutor:
- Does this person think I will steal their phone?
- Does this person think I will break their phone?
- Does this person think I have bad hygiene?
- Does this person think I will snoop on their data?
- Does this person think I am unable to use a phone?
If you keep standing your ground where you would try to explain this refusal by rational thinking, while it truly is emotional, you risk to reinforce the feeling that you are hiding one of these negative, insulting reasons.
Regarding your first question, I admit I had few opportunities to realize my friend would not let anyone touch his phone simply because he kept it very private. Avoiding showing things on it delayed a lot the time I asked for permission to touch.
Regarding your second question, I would imagine that, once you have to deny authorization
- Apologize for not letting them touch. If you admit that it's you, not them, you disarm the possible insulting implications. This is probably the most important.
- Provide a reason that looks plausible and not insulting. I don't think there are many of these, but some people might be more creative than me in that area. You could stay vague and say "I don't like people touchinftouching my phone".
- Move on smoothly. Start conversing where you stopped previously.