Paraphimosis: This condition is characterized by the retraction of the foreskin behind the glans penis, which can cause swelling, pain, and redness. The presence of a constrictive band of tissue behind the glans in an uncircumcised male who recently became sexually active strongly suggests this diagnosis. The condition is an emergency that requires prompt treatment to prevent complications such as ischemia and necrosis of the glans.

Balanitis: An inflammation of the glans penis, which can be caused by infection or irritation. It's common in uncircumcised males due to poor hygiene, leading to bacterial or fungal infections. Symptoms include redness, swelling, and pain, which align with the patient's presentation.
Phimosis: A condition where the foreskin cannot be fully retracted over the glans. While it doesn't directly cause the acute symptoms described, it can predispose to paraphimosis and may be a contributing factor.

Penile Fracture: Although less likely given the description, a penile fracture is a serious condition that requires immediate surgical intervention. It typically occurs during an erection, with a sudden bending or trauma to the penis, leading to a "snap" or "pop" sound, followed by pain, swelling, and deformity.
Epididymitis or Orchitis: Inflammation of the epididymis or testis, respectively, which can cause pain and swelling but typically involves the testicles rather than the penis itself. However, the pain can sometimes be referred, and these conditions can be serious if not properly treated.

Penile Cellulitis: A bacterial infection of the skin and subcutaneous tissue of the penis, which can cause redness, swelling, and pain. It's less common in the context provided but should be considered if other diagnoses are ruled out.
Fixed Drug Eruption: A rare condition that can cause lesions on the genitalia, including the penis, in response to certain medications. The lesions can be painful and may resemble other dermatological conditions.