Short answer: It’s harder to pee during an erection for two main reasons: mechanical changes in the penis that narrow or kink the urethra, and nervous‑system reflexes that increase sphincter tone. The body does close the bladder neck during ejaculation (to stop semen going into the bladder), but the difficulty peeing during an erection is mainly a combination of anatomy, reflex control and behavior — not a purposeful ongoing protection of sperm from urine.

Step‑by‑step explanation

1. Anatomy — where the urine flows: The urethra (the tube that carries urine and semen) runs down the length of the penis. It is surrounded by erectile tissues: two corpora cavernosa on top and one corpus spongiosum (which contains the urethra) on the underside. During erection the corpora cavernosa fill with blood and become very firm.

2. Mechanical effects during erection:

• When the corpora cavernosa become rigid they change the shape and stiffness of the penis. This can compress or distort the urethral lumen in places, or change the angle/length of the urethra, making urine flow more difficult.
• The corpus spongiosum becomes turgid but less rigid than the cavernosa; it helps keep the urethra open, but doesn’t always fully compensate for the surrounding pressure and bending.

3. Nervous‑system control:

• Erection is driven by the parasympathetic nervous system (blood flow increases to erectile tissue).
• Sexual arousal and orgasm involve sympathetic activity that contracts the internal urethral sphincter (bladder neck). That reflex exists so semen does not travel backward into the bladder at ejaculation (retrograde ejaculation).
• Even before ejaculation, increased sympathetic tone and pelvic floor muscle contraction during arousal can raise sphincter tone and make starting a urine stream harder.

4. Psychological and behavioral factors: Anxiety, distraction from arousal, and conscious tension of pelvic muscles can all make it harder to relax and initiate urination. Many people find it easier to urinate once the erection subsides.

Does the body prevent urination to protect sperm? Not exactly. The bladder‑neck closure during ejaculation is specifically to prevent semen entering the bladder and to help propel semen outward. The difficulty urinating while erect is not primarily a deliberate protective mechanism to shield sperm from urine — it is a side effect of the anatomy and the reflexes of sexual arousal and orgasm.

Is this normal and what can you do?

• Yes — mild difficulty peeing while erect is common and normal.
• Tips: wait until the erection lessens, relax your pelvic muscles, try sitting down or leaning forward, think of something calming, or run warm water to help start the flow.
• If you cannot urinate at all and your bladder is painful/full (acute urinary retention), seek medical help promptly — that’s a medical emergency.
• If you regularly have major trouble voiding even without an erection, or you have pain, weak stream, or urinary infections, see a clinician — causes can include enlarged prostate, urethral stricture, or neurological problems.

Bottom line: Difficulty peeing during an erection is mostly mechanical and reflexive (tissue compression, pelvic muscle tone, and bladder‑neck reflexes), with a smaller psychological contribution. The bladder‑neck closure during ejaculation is intended to protect the direction of semen, but the routine difficulty urinating while erect is not a special protective mechanism for sperm.