Why Is Masturbation Taboo?

I’m in my late 50s and happily married. I have a good life, all is well.
Couple of things though, and not even sure if it’s that big a deal:
- I am a chronic masturbater; I actually like it better than sex. Physically I know exactly what to do to make me feel good. Mentally it’s basically the only situation where I’m not thinking about anything or anybody else, I am completely immersed in myself. My wife knows I masturbate, but has no idea how often and intense.
- At the same time I’d love to experience this with another man. I found out I get aroused and excited by people who are genuinely horny. Real people masturbating turn me on, no matter man or woman. I love my wife very much, but she’s not into this at all. I have no interest in ‘cheating’ on her with another woman. I’m not sure if masturbating with another man is cheating. Is it?
I would be keeping this a secret from my wife, she would never understand.
- I am not that far from actually doing this. I’ve secretly been going to a nude recreational area regularly since not too long. It’s obvious plenty of guys are looking for action. Nerves and potential guilt have so far kept me from getting involved, but I’m getting close.

I’m an open minded person, fully in support of Ltbqi+. I think a lot of progress is being made, but talking about masturbation or solo sex still seems to be a no go area.
It’s maybe my favorite hobby, I don’t do no harm, but bringing it up in a conversation is taboo. Why is that?
Do you like to masturbate? Do you talk about this?