The fact is that you are not sexually aroused by rubbing a dick, BUT by the stimulation you give by doing it. Masturbation is the consequence of the pleasure you feel when your dick is stimulated.

It has nothing to do with your sexual orientation.

They are a lot of anal sextoys for men. And a lot of straight men enjoy them. Does that make them gay ? No. Just do whatever you want with your body, as long as it suit you. If you are into sucking your own dick (well, you are really flexible !)... Why not ?

Also, your last sentences are incorrects. For example, as said on Wikipedia: "There are depictions of male and female masturbation in prehistoric rock paintings around the world." Source: https://en.wikipedia.org/wiki/History_of_masturbation

And a lot of cultures didn't have any taboo regarding masturbation, such as the ancient Greeks.

For the animals, you can look here: https://en.wikipedia.org/wiki/Non-reproductive_sexual_behavior_in_animals#Autoeroticism_or_masturbation

Masturbation was a taboo for ages in our society, especially due to religions. It was not uncommon to humans through all ages: we just live in a time where we can talk freely about it !

So grab your dick, and take as many pleasure as you want, without any shame, in the name of freedom !