How can semen be pure when it exits from the same opening as urine and mixes with residual traces of urine in the urethra, thereby becoming impure?

The preponderant opinion is that semen is pure, and its emission from the urinary tract does not render it impure. Scholars have mentioned that semen has its own specific duct, and even if it were to exit through the urinary tract, internal contact does not affect its purity. The passage of semen through the urinary tract is not an indication of semen's impurity.