Thanks for your query.Based on the facts that you have posted it appears that you have repeated episodes of UTI due to unprotected sex
Though your tests for STD
are negative possibility of developing stricture of the urethra
or Chronic urethritis
secondary to UTI can not be ruled out
Please consult qualified Urologist
for assessment .You may need to under go radiological test like Urethrogram to rule out the same.