I think the reason the male-female couple with a bisexual gf thing is the most, I don't know, socially acceptable? triad formation is because, sadly, the world is dominated by hetero men. In the gaze of the Old White Male, it's ok for a female to sleep with another female but not another male, its ok for a woman to be bi, but not a man.
Sometimes the couple/gf triad occurs organically,and sometimes, yes, people seek it out. A couple seeking a girlfriend is no less ridiculous than a single woman with kids seeking out a single man of a certain age and income bracket who will love her kids and has qualities like good sense of humor and must love dogs...Will she get exactly that guy, and even if she does, will he be the right guy for her? Who knows. They say the universe gives us what we need, not what we want. Relationships take on a life of their own, and those couple/gf triads either fall apart or evolve, just like any relationship, monogamous or non-monogamous.
But why wouldn't people on this forum, especially those who have gone through it on one side or the other, not want to help these people in this situation? No one gets frustrated dealing with the same issues over and over again from mono partners dealing with jealousy for their poly partners, or people trying to move from cheating to polyamory. Why not show the couple/gf triads the same empathy and compassion? They're at a place in their journey.
As for the labels, they bug me. "Unicorn hunters" and "swingers" are looked down on, who gets to decide this? I remember when I first came on the boards, I described the woman I was sleeping with outside of my marriage as my "mistress. " People on the board promptly "corrected" me. "She's your girlfriend," (well, no, she wanted no responsibility or attachment.) "Call her your paramour." (What is this, Versailles circa 1809?)
We all come here because we want relationship advice for the kind of relationships we likely didn't ever see growing up. We are all trying to figure out how to make this work for us, and have more love/sex/passion/companionship in our lives. Let's embrace the full spectrum of that.