Because it's not about you. The Golden Rule is not "call people by the name you are ok with if you were them", it's "call people by the name they want to be called by". If you like, I can pull out my Miss Manners book and quote that rule to you, from the 1950s.
And in the specific case you mention, I'll say as a gay marriage advocate, that someone who doesn't believe in gay marriage should not be forced to use terms they believe apply only to heterosexual marriages. IMO both of those people are entitled to their views and preferences and both need to agree to disagree. Without being disagreeable.
The problem is that people read way too much into what other people say. It sounds like you hear "partner? this person is clearly insensitive to the history of discrimination again LGBT people."
Instead try "I don't like the word partner, but it is a pretty standard term for not only the unmarried, but also married people. This guy probably doesn't understand it bothers me, but I'll give him the benefit that he meant no harm."
And so is my gay friend in NYC. It's just that he's scared that his boss is actually a passive-aggressive homophobe and he's going to get fired for politely insisting on "husband".
He's just scared.
Keep in mind that he spent the first decade of his career working in places where he could be legally fired for being gay.
And there are lots of parts of the US where he could still be legally fired for being gay.
On the other hand, if you're OK with being respectful to people even if you disagree with their very existence, you're pretty much good to go in the Bay Area.
As an example of this in action, I have a gay friend who eventually moved to New York City with his husband. He works in a big bank in a mid-level job, you'd recognize the name. His manager constantly refers to my friend's husband as his "partner". My friend thought he was going out on a limb when he corrected his manager with "Actually, he's my husband." It's fine to be forgetful; it's not fine to be disrespectful. So far nothing bad has happened, but who knows how it will turn out.