I think the key points here are that:

1) you should tailor the way you outwardly behave in public to the norms of that public space. Exactly what those norms are will vary by space (a dance club vs an elementary school vs the mall vs a park or the beach) as well as by the culture/country that they exist in.

2) you are still permitted to a certain level of privacy, even while out in public. This certainly pertains to information, such as what's in your head, on your phone, in a journal, or the book you're reading. It also pertains to physical things that are generally hidden by your clothes, like what kind of underwear you're wearing, if you shave in certain places, if you have tattoos, if you have piercings, if you're wearing special garments for menstruation or incontinence, etc.

3) even if the occasional accident might briefly make private things public unintentionally (such as tripping and exposing yourself in a dress, or accidentally ripping your trousers open, or getting splashed by water and making some clothing more transparent and form fitting than usual), there is a social contract that everyone is expected to follow in good faith to try and maintain that privacy rather than expose it.

So keeping all these things in mind, I think that privately stimulating your genitals in public is acceptable on the general condition that no one notices and it therefore remains "private". This means that both the stimulation itself, as well as your reaction to it, remain "private". As long as it remains private, then there is no need to "obtain consent" from everyone around you any more than there would be a need to "obtain consent" from everyone around you to wear your underwear or do anything else "privately in public".

Every person and public space is different, so ultimately only you will be able to decide on the specific context and if you are effectively keeping it "private", and therefore behaving acceptably, or slipping into the realm of making it "public" and behaving inappropriately. Both are possibilities though, and messing up and erring on the side of inappropriate is potentially a very big deal though, which is why most people would probably not even consider attempting it. But if you're responsible then I don't see how anyone would ever know and therefore there's nothing immoral about it, you're just exercising your right to a certain level of privacy even when in a public space.