Some religions and moralists consider the act of sex or masturbation to be immoral, or a "sin." As a byproduct, members of such institutions, for example Catholicism, may feel guilt when engaging in consensual sexual activity. Thus, many individuals and organizations find ways to avert guilt, such as confessional in Catholicism, or the "born-again" movement in Christianity.
The phrase "wash away the sin" appears to originate from the act of baptism, prevalent in Christianity. Here, referenced to in the 19th Century gospel hymn, "Nothing But The Blood":
"What can wash away my sin?
Nothing but the blood of Jesus;
What can make me whole again?
Nothing but the blood of Jesus."
Urine acts as a dilutant to clear the urethra of any lingering seminal fluid and/or semen, thus "washing away" the "sin." Oftentimes, this act may feel pleasurable to the individual.
Tim: "How was your bone session with Chloe last night?"
Steve: "Great. But afterwards, I had this deep-seated urge to wash away the sin."
Tim: "I don't blame you, Steve. Otherwise, you might've come down with doom dong!"