one could reason that because all of the social classical conditioning society has been put through, society now looks upon down upon anything that is strange or different, anything that isn't perceived as 'the norm.' historically, the bible has had a lot of influence on the way we view things. homosexuality, bisexuality, etc. are viewed as strange by the majority of the public because the presence of the the bible and other major religious works; they have conditioned society into believing that homosexual/bisexual relationships are wrong, by describing all perfect relationships as heterosexual and monogamous.
i think now as society becomes more liberal and free-thinking, restraints on the 'rights' of sexuality will lessen and one day it won't matter what gender a person prefers because like hetero relationships, homo/bi relationships will have joined the ranks of what we view as normal. people are mostly afraid of these kinds of topics because they're ignorant. in my experience, men are more ignorant about what it means to be gay than women are. most men aren't educated about what it is or are uncomfortable learning about what it means to be homosexual, so they perceive rumors and lies as the truth which further distorts reality. perhaps if everyone learned about lesbian and gay relationships as well as straight early on than this social awkwardness regarding sexuality wouldn't be so much of an issue?