Side effects of medication, specifically the Fluoxetine and Propecia, can cause a loss of sex drive. I would also consider a testosterone level to make sure these levels are normal.
Regarding the irritation and burining - I would test for infection. Specifically, STDs such as chlamydia or gonorrhea can cause these symptoms. A U/A as well as urethral swab for the STDs would be a good start. Digital rectal exam as well as scrotal ultrasound to evaluate for prostatitis or epididymitis can also be considered.
Followup with your personal physician is essential.
This answer is not intended as and does not substitute for medical advice - the information presented is for patient education only. Please see your personal physician for further evaluation of your individual case.
Thanks,
Kevin, M.D.