Here is a link to a post where people were discussing some of the terms that illicit negative feelings/responses. It's a few pages, but the first few post might give you an idea of why certain terms raise some red flags for people.
In theory, there is nothing wrong with having a closed triad. However, more often than not, the "unicorn" or "hot bi babe" is the one that gets the short end of the stick.
Personally, anytime people seem like they are trying to fit real people in the real world into their fantasy - I see red flags as do many others and therefore they go into warning mode. Ah, wait a minute, back up, are you really meaning to say what I heard you say...