I don't think "orientation" is that clear cut. Most people have found themselves attracted to someone of both sexes at some point. This common question stems from that. For instance, many men admit that though they consider themselves "straight," they have found some obscenely attractive male star at least slightly (and usually uncomfortably) arousing. Many women also admit to finding some celebrity women attractive. I think people are most comfortable joking around about attractions to celebrities because they feel distant from them and nothing would really ever happen anyway. It essentially makes it feel less "gay," which is especially important to the male ego.