It's been my experience that bisexual males are much rarer than bisexual females.
So, you'd have to have not one, but two bisexual males to make a triad. If HBB (hot bi babe) is a unicorn, I'm afraid to venture a guess as to what HBD (hot bi dude) is!
I really don't think it's an insecurity thing on the part of men. If you're not bi, you're not bi ...