As a switch here I can tell you that some of the reactions I have gotten are bizarre. I have been told that I am just "confused" "not serious" or really just need the right Master. I've also been told by dom/mes that they won't go with switches because "they always try to flip you". And it is very parallel to the reactions I get as a bisexual in other fora. Not to say that this is the most common reaction, but it happens often enough.
I think many of the ideas others have brought up are quite valid, but there is one that I don't think I have seen yet, and that is performance anxiety. Once (not here in the library) I had someone tell me that they would not ever think of topping me because they were afraid that I was a more skilled domme than them and they wouldn't measure up.I've gotten the impression more than once that some people are afraid that a switch will be judging them against the switch's own performance in that role, and I have never figured out quite how to address that.
And to sinderalla, I understand you quite completely. For me, my switching is absolutely gender-based. I domme men and I sub to women. For me, that's the way my fetishes run and I can't explain it any more than I can explain why leather and ropes get me wet.
I can somewhat understand Timoty's point about a switch not being being his completely. But only in the context of a long-term monogamous situation. The same thing happens with bi's, so many lesbians will not date a bi woman because they assume that she will eventually turn to men again. Of course what he is failing to realize is that people can commit to the right person and stay committed no matter what outside distractions there are or what form they take. Bi's can stay in monogamous relationships no matter what gender they are with, and switches can commit fully to a D/S relationship no matter what side of the slash they are on.