Well the bisexual and asexual being more strangely looked upon kind of makes sense if one puts themselves into the mind of a narrow-minded person who likes conventionality and stereotypes.
1) People love boxes and categories.
They make them feel secure, give them something to identify with and make it easier to blame someone from the other 'box' for something that's usually their own/their boxes fault. Bisexuals straddle both boxes thus confusing the line. This scares people (a bit like biracial children scared people some decades ago). The 'how can you be both' makes people unable to relate. For asexuals this question is the 'how dare you reject categories' or 'how dare you have the best of childhood (free from sexual pressures) and adulthood (being a book buff)
2) Being able to relate
- many friendships come about and survive by 'clicking' i.e. sharing similar life stories, experiences and values/opinions. If you look at most gay-straight friendships it's usually gay man-straight woman or straight man-gay woman. This is because it makes the appeal of having an opposite gender partner and the common points of having a same gendered one merge in one person. E.g.- a camp gay man and a straight woman can chat about which colours match and stare at Brad Pitt for hours (as well as diss patriachy). A (butch) gay woman and a straight man can stare at Megan Fox and be the best beer buddies.
3)Not feeling threatened (and good old steretypes)
- a straight woman doesn't feel her gay friend will hit on her or try to steal her husband (supposing the husband is straight
) and vice versa with with straight men and gay women. This is different with bisexuals. Everyone thinks they are either: going to steal their spouse, hit on them or persuade them to a threesome (because of the promiscous sterotype). For asexuals people will think: they are boring, no chance of having 'benefits' with that one, they may not steal my husband but they totally won't understand how annoying that sideways position my partner insists on is, how my condom broke last night and that's why I am so stressed today (or haven't had had any in a month feeling cranky), they are sissies,etc
4) Subverting conventional relationship models (i.e. monogamy)
- bis and aces are more liekly to form polyamorous relationships and thus blur the line of friend vs partner which again makes people uncomfortable and confused about what to do.
PS Read 'The Colour Purple' by Rebecca Walker who was a biracial bisexual woman living at the pinnacle of Civil Rights movment and seuxal revolution and how she had to fight discrimination, misunderstanding and fear from all sides of her own identity. Fascinating!