Everyone has the right to NOT support a company based on any reason. It's your money. However, again, it seems highly impractical to boycott an entire company because it has an employee that supports something you object to. So where are you drawing the line. If Starbucks has someone at the corporate level (not CEO), that supports anti-gay bills, are you going to boycott the entire company then? What about all the other employees they have that you don't know about, and who technically are indirectly funded by you when you buy a product, who also support things you object to?
Or do you only draw the line when it's someone that is the head of a company?
You can do whatever the hell you want. But I personally see this as highly impractical. And I think if you are going to use this line of logic, you better be prepared to boycott a lot of things you buy from any company that has over 1,000 employees.
Because people are trying to make a public statement, not change how the world fundmentally works. Once again it's the "If I can't stand up for everything, I shouldn't stand up for anything." argument and it's absurd because if everybody took that attitude nothing would ever change.