Okay, coherency time. You may already know this one, but in the poll described in this article (http://www.participations.org/Volume%205/Issue%202/5_02_pagliassotti.htm), 47% of BL manga readers identified as heterosexual in an English-language survey, and 62% of BL manga readers identified as heterosexual in the same survey when it was done among Italian-speaking readers. It's about BL manga, not slash, but pretty interesting in light of your findings. (I should really try to find data about how much overlap there is between slash and BL manga readership. This must have featured in at least a couple of surveys already...)
The following is just speculation based purely on my own personal experience. I think it's possible that numbers in these polls got a little bit skewed in the direction of 'queer' because a) the polls were taken in an environment where admitting queerness is not generally a problem, as others have already mentioned and b) people like to make their voice heard, and if a poll offers them a chance to make their voice heard when they can do that only infrequently IRL, they might rush to participate. People who identify as mostly or entirely heterosexual might not feel much need to affirm their identity. Me on the other hand, I love ticking the little box besides 'bisexual' so much, because I don't get to tick that box every day. It's kind of a way to yell "Hey, I exist, there's bi people in this fandom!". If a poll offers me the opportunity to tick that box, I'll feel happy and be much more inclined to finish it than if it offers only, say, 'heterosexual' and 'homosexual' as choices. So perhaps heterosexual fans might not flock towards these polls with quite as much enthusiasm as fans who identify as queer. By which I don't mean to call into question the basic point of your post -if the numbers are skewed towards 'queer', it certainly won't be by much.
no subject
The following is just speculation based purely on my own personal experience. I think it's possible that numbers in these polls got a little bit skewed in the direction of 'queer' because a) the polls were taken in an environment where admitting queerness is not generally a problem, as others have already mentioned and b) people like to make their voice heard, and if a poll offers them a chance to make their voice heard when they can do that only infrequently IRL, they might rush to participate. People who identify as mostly or entirely heterosexual might not feel much need to affirm their identity. Me on the other hand, I love ticking the little box besides 'bisexual' so much, because I don't get to tick that box every day. It's kind of a way to yell "Hey, I exist, there's bi people in this fandom!". If a poll offers me the opportunity to tick that box, I'll feel happy and be much more inclined to finish it than if it offers only, say, 'heterosexual' and 'homosexual' as choices. So perhaps heterosexual fans might not flock towards these polls with quite as much enthusiasm as fans who identify as queer. By which I don't mean to call into question the basic point of your post -if the numbers are skewed towards 'queer', it certainly won't be by much.