I guess another way to look at it is that you should try making a compromise. If you wash your hands in front of her, she should in turn wash her hands. If that doesn't work out then tell her that you are going to have to switch keyboards because you are in love with your newly acquired stuff. Hopefully she will let in otherwise yeah the arguments will definitely ensue and may not necessarily be a good thing for relationships.
There is another way but that requires one to be in a more calm and collective state in both mind and when speaking. By saying that I would almost sound like a Zen monk trying to recite words of wisdom (or even Cesar Milan himself
) ). Evidently it is easier said than done for situations such as a brand new and fairly expensive keyboard is on hand and that one with grubby hands wants to try it out regardless.
Anything at least $50 to $100 and beyond for just a keyboard instantly goes into my "expensive and fairly treasured items". However in saying that it is much easier to raise one's voice in which that can and will cause an argument over views. However if one can practise an ideal approach of dealing with virtually any situation it can go a long way. I too am personally trying to practise the notion of being "calm and assertive". I admit I can get hot headed when someone persistently refuses to follow and accept what I am trying to get across. Then again the ideal approaches in life is to try and to not raise voice early in some dispute because it is a simple tell-tale sign to another person that you are getting angry with them.
Recently I have been in touch with my former self (long story - don't ask) I started watching Cesar Milan's shows (via Youtube) even though Cesar Milan is no god or some sort of deity, he does say certain things that maybe useful elsewhere apart from handling dogs (mainly dogs, not sure about cats for instance but it may also work - just don't quote me on cats). The phrase of "calm and assertive" was a quote none other than by himself as far as I am aware of however in this
thread (some other site that is of course) it seems to show the same set of apporach may work elsewhere.
For the record, I too would be annoyed if someone had grubby fingers wanted to fiddle around with my newly acquired expensive item. Regardless if the item is HHKB or Korean custom keyboard (which I forked out large money on) or even just an expensive mechanical keyboard. The case does not apply to just keyboards, virtually anything that is worth a fair amount of investment I would like to assert some rights over just like how a mother were for her newly born offsprings (in a sense of being protective). I am also not that great when it comes to saying the right words at the right time (only vocal, not when writing in text) but if somehow I could practise to link the way I speak via text or via voice is the same I maybe able to handle situations better.
Getting hot headed under issues is easy, resolving issues the right way is often (not always) hard. Maybe your girlfriend may pick up a hint or two (after cooling off from the argument for about a day or so) that her actions and response maybe wrong if she thinks of other possible reasons as to why you were angry with her and her grubby hands over your new keyboard. You could go as far as saying that you paid $300 for the keyboard which might even tell her that you are serious with cleaniness.