Because they are bringing other people into their unhealthy dynamic that is why. I couldn't care less what a couple do between themselves but she is advertising for an equal third partner after finding a V doesn't work for them, why didn't the V work I asked myself? So I read this thread, because she was jealous and felt left out.....their response? Find a flaming Unicorn. Yes that is a jolly good reason to be hard.
If it was stated in the advert that this might be entirely experimental on her part, I would not mind so much but it doesn't and what happens to the sparkly Golden Unicorn if she finds out that a) She can't love another person and B) That she is not Poly?
I think that is a very valid question to ask.
I can't speak for the OP'r, but my feeling around her situation is that her husband wanted to open up their monogamous relationship. She loves him and has struggled with her feelings around this new dynamic. She is asking why SHE is the one that needs to do all the emotional work to save their marriage, versus her husband that can easily slip into polyamory. So, she tries to explore some of her ideas/fantasies with bi-sexuality and making it all work (yes, with a unicorn).
So the struggling 'mono-wired' spouse is the one at fault? I see her doing more work (from what we've been presented) to find a solution than her partner here. He just gets what he wants and she is left struggling with her emotions around it.