I think each admission of ignorance is an opportunity to educate. Screaming at people about how wrong and offensive they're being, whilst tempting, really doesn't fix the problem.
I agree with this, too.
But some of the best people to educate the OP are sufferers of the illness, their relatives and partners and people with professional knowledge. The last of these groups would probably just roll their eyes at the ignorance, they encounter it every day. But the people who have bipolar disorder, or care about others who do, mind terribly about the stigma around this illness.
These people are likely to be alienated by what the OP said, his concern that the illness might make someone a rapist (because this is what sexual 'coercion' means isn't it?). So it's not a very sensitive way to go about seeking useful advice.
It's not a question of people choosing to be offended, they just are. What he said is objectively offensive.
And yes, what everyone says about minding his own business about his ex's sex life. This is separate from his ignorance about mental illness, but would apply in any case. If he is genuinely concerned out of caring for his ex, why the hell is he only asking about the sex?
It's odd, it's creepy and it's inappropriate - definitely. And is he using her new partner's illness as an excuse for plain old stalking behaviour - maybe.
And this would apply if it was a man or woman who'd started the thread.