I had unprotected anal sex. i got burning urethra. no other symptom. urine test negative. taking antibiotics for possible uti. now semen is thick. why?

STD?: Did you tell Dr you had a sexual exposure? Urinalysis does not identify most STDs / causes of urethritis (=burning urethra). Most common Gonorrhea & Chlamydia. Likely will need more specific testing. Semen viscosity varies by %s of other components in it, which are influenced by "prep" time--need time for pre-ejaculate secretion to have "thinner" ejaculate. STD prevention easy with CONDOM use.