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The body is designed in a way that the sperm and urine do not mix even though the outlet is the same. There is a Y junction inside guarded by a sphincter.
Don’t blame on masturbation
, as it is not harmful.
The white color substance is called smegma
. It will soon disappear when you start having sex regularly and clean the area during bath.
If rashes and itching are persistent, it is worthwhile consulting a doctor.