Maybe the cat thinks you dirtied up her clean tongue by touching it and she cleaning her tongue? My cat rarely yawns so I can't try it out. The cat is more of a "talker". Normal what happens when my cat or dog, (or even a parrot), yawns is that I yawn too. I believe cat saliva is supposed to have some healing properties, (along with other yucky-stuff).

I'd probably want to wash if someone unexpectedly touched my tongue, too - you know, personal space and all that!

Most of the cats I've known would just bite you.