Is this abnormal behavior (maybe OCD) after masturbation?

First of all semen ( which contains sperm) is not filthy. It is a natural substance made by your body. Neither is masturbation dirty in any way. It takes a lot more than just a drop of semen (sperm) to give off an odor that can be noticed, even in a closed bathroom. As long as you clean yourself up and no semen is visible anywhere near you, you have nothing to worry about. If you get semen on yourself a shower is ok. Just stop obsession over it. You seem to have the idea that semen is some sort of bacteria that needs to be sterilized. Not so! If your dad and mom thought as you do, you never would have been conceived and born. Also this world would not have 6.5 billion people in it. The human race would have become extinct long ago I suggest that you are being paranoid.