Being a CEO very rarely means you have free reign to be an ideologue. CEO is a management position, not advocacy. Personal beliefs can and do tinge a person's business decision making, and a person in such a public-facing position as a CEO needs to temper their opinions. That's the reality of it.
You do not have a right to be a CEO of a corporation. This is basic corporate rights [determining who is an isn't able to be their CEO] and action/reaction to PR tied in with societal events [ousting him due to a few reasons other than simply anti-gay donations]. Noone in the public forced him to resign, he resigned to save face for the board who shoehorned him into the position and realized they made a pretty big mistake after half the experienced board packed and left in protest.
This has less to do with his meager donation and the fact that a public-facing position does not need to be overshadowed by a person's beliefs and past, which it was. Eich was playing this appointment and role for his own image and actually trying to play anti-gay at the same time:
Eich also stressed that Firefox worked globally, including in countries like Indonesia with “different opinions”, and LGBT marriage was “not considered universal human rights yet, and maybe they will be, but that’s in the future, right now we’re in a world where we have to be global to have effect”.
That's why you typically pick qualified uncontroversial candidates from the beginning [but, in this case, Eich is a friend of the director of the board and was allowed to be CEO with no ability to do the job], to avoid scenarios like this. And especially ridiculous quotes like the above. He was not suited for the position in the first place and his resignation in the face of a moderate controversy [which would've been better solved by him, I dunno, explaining himself instead of playing the victim and acting like teh gayz forced him from his position] just solidifies his inability to be in that position.