If someone says to you "I'm gay (or lesbian)," or "I'm bi-sexual," is it rude to say "I don't care"?
My initial impulse was "No," but context is everything.
When I say "I don't care," I mean it in a neutral voice. I just don't care.
If you're a woman and trying to hit on me, I'm very flattered, but you're barking up the wrong tree, and we only have a problem if you push it. However, in that case - same as if you were some man who might think he's God's gift to women - the problem isn't your sexuality, it's your aggressiveness. (Unless you're also Angelina Jolie hitting on me - everybody gets a pass if it's Angelina Jolie.)
However, there is a contextual element to be considered: for many people who are gay or bi-sexual, the realization and public admission is very threatening, and the act of saying something can make the person both emotionally and physically vulnerable. (It's a sad commentary on human nature that it makes them physically vulnerable, but that's a different rant.)
So, maybe I should say: "Thank you for sharing that with me, I'm sure it has added some real challenge to your life that straight people don't suffer (and no one should have too), and I empathize with that, but otherwise I just don't care."
I don't care. The only impact it has on my life is because a bunch of twisted-up titwits use it as an excuse to discriminate against others, and a bunch of fuckwits use that discrimination as a launch platform for all manner of power-grabbing political shenanigans. Click on the "class warfare" and "gay marriage" labels below to see how much these shenanigans piss me off.
So, I guess the morale of this tale is that rudeness is context sensitive. Even the kindest-sounding sentence can be the height of rudeness when delivered with a scornful tone, or at totally the wrong time. So, as always, think before you speak, and only be rude by deliberate intention, and not by careless accident.