Do you think you’d actually say that to a stranger in real life, though? Maybe to a friend or someone else you were close with who understands your perspective and you’ve already developed common ground and rapport with. And that may be part of the issue with Internet communication. You are essentially talking to someone (or challenging them) about very personal things, with minimal understanding of them, and they have minimal understanding of you and your intentions as well.

Or it is just related to the phenomenon of people willing to be more confrontational behind the safety of their keyboard (in both directions).

With regard to why someone took offence to what you said, consider this: if a 14 year old boy said he liked women, would you also question his certainty in his sexuality even if he had 0 practical experience? Or is it only something you’d question for bi or otherwise queer people whose preferences aren’t conforming to the standard? I suspect the latter, because it’s pretty damn rare for anyone to ask straight people if they are sure they are straight… shouldn’t they have some homosexual experiences first to be sure?