> Is it possible they are bi
Everything is possible, especially considering that Marlon Brando and Freddy Mercury, two men with 100% masculine manners, were bisexual. So, how can I answer NO!? :-)
More seriously I believe that many men and women are potentially bisexual, but they don't (want) know this, because they have strong preferences towards the opposite sex, and they have no interest into risking moral judgment from the society. Think to what happens in prisons between men, that probably will have no homo-sexual behavior or fantasies outside of it.
I believe also that a feminine manner or a masculine manner (within certain margins obviously) in a man, is not a a reliable way to predict his sexual choices, in the sense that there can be many exceptions.