Thanks for a very typical question.
The cause of you getting recurrent tonsillitis after kissing your boyfriend using tongue is that the bacteria which harbors in his throat may be commensals (may not be causing him the problems) for him but act pathological in your throat.
How to avoid:
No kissing at the moment.
A course of antibiotic for both of you.
If the next trial works without infection, well and good; if not both of you must consult an ENT Surgeon, get examined and tested and treated.
These Q&A’s are for educational purposes and should not be relied upon as a substitute for medical advice you may receive from your physician. If you have a medical emergency, please call 911. These answers do not constitute or initiate a patient/doctor relationship.