Proctitis: This condition, which is an inflammation of the rectum, is a common consequence of receptive anal sex, especially if protective measures were not used. The symptoms of rectal pressure, burning, and discomfort during urination are consistent with proctitis, which can be caused by bacterial, viral, or other infectious agents.

Anal Gonorrhea: Given the history of receptive anal sex, gonorrhea is a plausible diagnosis. It can cause rectal burning, discharge, and pain, especially during urination.
Herpes Simplex Virus (HSV): HSV can cause painful ulcers and blisters in the anal region, leading to symptoms like rectal burning and discomfort.
Syphilis: Primary syphilis can manifest as a painless chancre in the anal region, but secondary syphilis might cause more systemic symptoms along with rectal discomfort.
Bacterial Proctitis (e.g., Chlamydia): Similar to gonorrhea, Chlamydia can cause bacterial proctitis, leading to symptoms of rectal burning and discomfort.

HIV Infection: Although less likely to cause immediate rectal symptoms, HIV testing is crucial after unprotected receptive anal sex due to the high risk of transmission. Early diagnosis and treatment of HIV are critical.
Rectal Tear or Fissure: A physical injury from anal sex could lead to a rectal tear or fissure, causing severe pain and bleeding. This condition requires prompt medical attention.
Abscess or Fistula: An abscess or fistula in the anal region can cause severe pain, swelling, and discharge. These conditions are medical emergencies that require drainage or surgical intervention.

Lymphogranuloma Venereum (LGV): A less common sexually transmitted infection that can cause rectal symptoms, including pain and discharge, especially in the context of receptive anal sex.
Rectal Cancer: Although extremely rare in a 30-year-old without other risk factors, any persistent or severe rectal symptoms warrant a thorough evaluation to rule out malignancy.