People don't believe you. They think it's some mythical configuration that went extinct in young people during the last cycle or two. When I tell people I'm straight I'm sometimes saying that there's a man or trannie/crossdresser I find attractive sometimes, but it's rare enough where if I'm to go as bisexual it would make it difficult, so I go as straight. Ultimately if you ask someone who knows something about how sexuality works they may be able to tell you more about how personal consideration is often woven into what it means for people