They are out there, looking for the HBB (hot bi babe), the perfect woman to "complete their family". Usually, there's quite a list of qualities that this woman should possess, as well as an even longer list of disqualifiers. The couple looking will remain the ultimate priority, of course, and any woman that emotionally or sexually threatens either partner will be out the door. Did I mention the expectation that she'll be sexually exclusive with just them?
Oy vey! There are quite a number of threads on places like OKCupid regarding the unicorn hunters. Many pertinent observations have been made, but the one that I'd like to focus on for this post is that the vast majority of the UH's are so green at poly they'd blend right into any hardwood forest without disturbing the trees. If someone starts talking with me about their desire for a unicorn, I can pretty well ascertain that they are likely to be rather new to ethical non-monogamy.
Now, let me be clear, there is absolutely nothing wrong with wanting to date a woman, even as a couple or group, what tends to be a problem is that this strategy is being used to assuage someones insecurities at the very beginning stages of exploring polyamory. It would serve all parties better to deal with the aforementioned insecurities before tangling anyone else up in that mess.
Let's say you're new to poly. Here's a nice hetero couple, and the woman thinks that she might be interested in exploring bisexuality. She'll be doing it with her male partner, of course. This is about bringing their relationship closer together, right? The man is all over that idea! It plays right into his favorite porn fantasy, plus, there's no messy territorialism to work through with another man. "His" woman will be untainted by another's cock, plus he gets to enjoy the variety of strange pussy, presumably while his partner gets something out of the whole equation. There's just the small issue of the unicorn in question being a person with their own thoughts and ideas, wants and needs, which may not include another man, another woman, or a group dynamic at all!
"But it says right here on your profile that you're a bisexual woman. Doesn't that mean you like both men and women sexually?" Sure, particular ones that are appealing in their own right. Not necessarily specific ones that are looking for a sex toy that will show up, fuck, and go back home when it's convenient. It is not anyone else's job to "bring you closer together". She may not be necessarily interested in a man and woman at the same time either.
What if she likes one partner, and isn't into the other sexually? Suddenly, this exploration of the female partner's sexuality has left someone out. Maybe she likes the man more, and the woman that was looking forward to this lovely sisterly bond, along with the grinding of girly bits, is left standing at the sidelines while "her" man is off cavorting with the object of her desires! Perhaps the woman is seen as more appealing, and the man is sidelined, feeling like the lioness failed to bring home the prey item as promised, largely because people aren't like antelope, they have more self-determination. ;)
While it is possible for everyone to fall in love across couple lines and form triad relationships that serve the needs of all partners involved, it seems like the place that many people start their journey into poly, unicorn hunting, is among the last places that are easy to explore in a healthful way for all concerned.
Start with solo dates. Feel the insecurities. Delve into the preconceived notions. Communicate and learn how it feels to be waiting up for your partner while they are out without you. Look into those scary places. Approach each new potential partner as an individual, worthy of being known as a person, not someone to fill a job description. Acknowledge that your partner might be interested in someone that does absolutely nothing for you personally, and that's going to be alright. Watch your partner fall in love with someone else and don't try to control their emotions by inserting yourself in their dynamic. Gain self-knowledge and apply it!
After going through this process, if you STILL really want to find a woman that will be interested in you both to date, start looking, patiently. Consider the idea that she will almost certainly have other partners as well as the two of you, probably other partners that are just as important as the two of you are to each other, maybe even children and/or a profession that occupies a good deal of her time and energy. She isn't likely to be waiting by the phone for you to call, ready to trot over as soon as your kids fall asleep, and will likely require real get-to-know-you dates with each of you separately, as well as together. There may be a stronger bond with one partner than the other. Allow the connections to be what they are, without trying to force it into a certain mold. Unbelievably, this seems to be the fastest way to find what you want. Stop trying soooooo hard, and become the type of partners that you would want to date yourself.