To be fair, it may not be something that comes up during a normal conversation. It’s really none of our business what other people’s preference in the bedroom is, so it isn’t something most people pry about.

I lived under the same roof (uni dormitory with 5 others) with a person for 3 years without knowing she was bisexual. It was only because she mentioned a female name in a subsentence about a tinder date that I found out about it. Unless they got a partner or you’re really close to them, it may just be something you never find out about, so you just assume they’re straight.