I think you're missing a key distinction. The problem is not that merely that you hold a political belief-- as you say, this by itself is over-the-top and unreasonable.
The problem rather is that your particular political belief is viewed as being actively hostile towards people themselves or towards their friends or family or others. Voicing opposition towards gay marriage comes across as an attack on all gay people. That's where the reaction comes from.
A political belief that's not treated as openly hostile towards others (for example, supporting charter schools or something) doesn't get this reaction, of course. It's not merely that "you just can't say that you believe certain things are right/wrong if it goes against norms"; you're missing the nuance here.
The issue is that with just a little bit of lather, it's possible to construe virtually any controversial opinion as "an attack" on the people that may be affected by it, however tangentially. Nothing in life is free and it is rare that there is a clean "perfect compromise" waiting to be discovered, which means that most trade-offs will have groups that can interpret the activity as hostile, even if their group is totally unaddressed ("Why does group Z get that thing and group Y only gets this thing?").
If we accept this argument that some opinions or positions are unconscionable because they are fundamentally "against" other people, we end up on a slippery slope, and the situation in SF is somewhere on that slope (in my opinion, nowhere near the bottom).
Everyone wants in on the sympathy train and everyone loves shaming and silencing their opponents. We have to be careful to actively protect civil dialogue from such encroachments, because while they may feel sympathetic and nice at the time, that self-righteous reward system gets put on a feedback loop and drowns out everything else.
If we've already seen people getting agitated into conflating arguments over the semantics and technicalities of a dry legal instrument like marriage with their individual right to self-existence and safety, that's proof of this phenomenon in itself.
The problem rather is that your particular political belief is viewed as being actively hostile towards people themselves or towards their friends or family or others. Voicing opposition towards gay marriage comes across as an attack on all gay people. That's where the reaction comes from.
A political belief that's not treated as openly hostile towards others (for example, supporting charter schools or something) doesn't get this reaction, of course. It's not merely that "you just can't say that you believe certain things are right/wrong if it goes against norms"; you're missing the nuance here.