I would have to agree with doogs generally and not agree with zip on this.
This isn't the privacy of one's home and you never know what else this person might be doing. Hopefully it is pretty harmless but how would you feel if this person was doing very bad things to innocents and this was just the tip of the iceberg.
Let's face it, it is outside what should be accepted. He should at least be investigated.
I have seen quite a bit of public masturbation - but always from mental illness or similar ------- this could be quite different.
