Mostlyclueless, think about it - the reason it's even called unicorn hunting is because it's so unrealistic and the young women often sought out to fill that role are called unicorns because they are mythical creatures. It's not that great relationships can't happen among three people, nor that there aren't any single people looking to be in relationship with a couple, it's that the unicorn is a ridiculous and pretty much impossible ideal to realize.
The animosity/frustration/low tolerance/ridicule you see directed at unicorn hunters has to do with unrealistic expectations that many newbie couples have about finding a person (preferably the "hot bi babe" or HBB) to fulfill a role that is focused on satisfying their desires, as if the couple is one unit, not two individuals, and the unicorn is expected to "join their relationship" instead of exploring the four different relationships that can evolve separately over time among three people.
Anytime you put the role first and try to fit a person into it, it will most likely fail simply because doing so negates individuality and a natural progression in relating to others. The stereotype is that the HBB love both partners equally (as if you could order your love like a sandwich), move in with them, share in household chores and childcare, service both of them sexually, and not have any other relationships with anyone else but them. Often the unicorn they seek would be a little younger, and less established, so that encourages some dependency on them (and some couples have been burned by winding up with someone who is just desperate and using them for a place to stay).
The unicorn will probably never be acknowledged publicly, nor have any say in how the household is run, but will be dumped the moment some stress or jealousy (or whatever) affects the couple (what I call The Holy Dyad). Some of these unicorn hunters won't even allow communication to happen between the poor unicorn and just one of them - there was one woman who came here and explained how all her emails, texts, and calls (they did not live together) had to go through the wife and she was not allowed to speak to the husband directly, even though she was expected to service him sexually. Unicorn hunters are all about preserving The Holy Dyad at all costs, and most every person who has been a unicorn comes here (and to other forums) and shares how they were mistreated and/or dumped without warning by the couple that promised them so, so much.
For some reason, most unicorn hunters seem to feel great disappointment if the triad starts turning into a vee - which often happens when their unicorn has a better relationship with or is more attracted to one of them and doesn't want the obligation of being with the other. So, instead of letting things morph naturally into something else, they get rid of her and start hunting all over again. They feel they MUST date as a unit, with no autonomy, otherwise it's a threat somehow.
It's a stereotype on both sides, for both the hunters and their prey, and of course there are couples who don't think this way. Unfortunately, however, there are still enough couples out there who live up to the stereotype and don't realize how unrealistic they are being with their expectations.
Maybe these threads will shed some light for you:
Added to, Joining In
Why Is it So Hard?
advice for couple seeking third
What's in it for a unicorn?
I Hope I'm Not Naive