I'm just of the opinion that I'm in no way obliged to disclose to you what I spend my money on, just because I work somewhere you spend money on.

...

So you believe that customers of a company should have a say how the staff uses their wages?

Can you see how these two statements are not the same thing? The former is not something anyone had said until you did so while the latter is what the entire rest of this conversation has been about until you tried to pivot to the former.

Indirectly yes a customer absolutely has a say in what an employee spends money on if they are aware of it because it's the customer's money that's indirectly paying for the thing. An employee is a part of a business and when an employee engages in some activity even on their own time if people are aware they work for that company, then that reflects on the company and vice versa. That's one of the reasons companies have very strict rules about representing yourself as an employee or agent of the company.

Furthermore the higher an employee's position in a company the stronger their actions reflect on the company. For someone like a co-founder, practically the highest position in a company, those actions reflect very strongly on the company. The only actions that reflect stronger are ones done directly by the company itself.

This is why HR departments exist, they protect the business from the actions of its employees (or at least attempt to).