That would have to go on my "most ridiculous things heard" list.
First, the penis doesn't fill with semen, the testicles do. The only time the penis fills with semen is while he's ejaculating. And even if it were the penis that held the semen reserve (which it doesn't), who's to say he isn't masturbating? Would bath water know the difference between semen released from masturbation vs. sex with a partner? Nope.
That's just too silly for words.