I have been with my husband for 11 years now, and I only have an allergic reaction to his saliva when he kisses my skin. It's also instant, red itchy and burns. Sometimes just his stubble on his face, that can give me the same reaction.
Oddly enough, I have no reaction when we kiss on the lips...or other intimate areas.
It's nothing to do with diet or toothpaste etc. If someone knows anything or has any ideas, I would also like to know. An allergist seems to be a good idea...maybe we'll try that next.