I think in the grand scheme of things, $100k isn't a lot of money for a company that posts BILLIONS of dollars of profits every year.
With that said, it is Apple's money. If you do not approve the politics they promote you can cast your "vote" with your wallet. Personally, I don't care what other people's or companies politics are.
I am a Republican but I will still buy Apple products because they are awesome. As a shareholder, I would base my decision to own Apple shares or not based on how much money they will make for me. I could care less whether or not Steve Jobs or any Apple executive or committee thought about politics.
In my Marketing education we learn about something called "Cause Related Marketing" where a company sponsors another entity or policy to leverage its association with it. It's possible this was a business decision to penetrate the gay community (no pun intended) too. Maybe Apple wants to align itself with liberal policies so that those gay people who need a computer are more likely to consider them on some level.
Another factor we must consider are political influences. Apple does business in California. A significant number of California lawmakers support gay marriage, many very passionately, and the ones who support gay marriage are in power. If Apple had donated money to the other side or stayed out of it, it is possible that they might have experienced trouble in the political arena.
They probably figured the $100,000 donation to their cause was better than risk problems with regulatory agencies, inspections, license boards, etc.
I also read somewhere once that Apple has a lot of gay employees and perhaps they wanted to keep their spirits high and those employees motivated and loyal to the company as well. The last thing Apple would want to do is anger a large base of their employees who may be motivated to leave or become less productive, both bad business outcomes for Apple.
In short, there are probably a number of factors that went into this decision for Apple, and I would bet its a combination of political leanings of the Apple executive board, potential political fallout, and some "cause related marketing" (not necessarily in that order and not necessarily in equal weight).
To not buy their products just because they made this decision I think is a bit short sighted. You are only hurting yourself really. At least that is my view. I wouldn't give up my MacBook and be relegated to using Windows again just to prove a point to people who don't even know I exist and will not feel the impact of my decision. With that said, its still your right to do so and if it makes you feel better then by all means.
(On an off-topic note, I never understood hunger strikes either. If someone ever did a hunger strike against me I would just let them starve. What do I care? They are doing it to themselves! But that's probably another thread.)