Language, terminology, and how you express what you want is very important. If you went to other sites saying you want to "add" a woman to your relationship, as in "add a third," I think that might be one reason you have been called unicorn hunters. Whenever people say that they want to add a person to the relationship they already have, it comes across as objectifying the woman who will be "added," as if she is meant only to serve the existing relationship.
Think about it, you add a condiment to your main course to make it more flavorful, it is meant to enhance the food. So, seeking someone to add to your relationship makes it sound like they are only there to enhance it, but it doesn't sound like the relationship you have with that person would be as important as what she could give to you and your existing partner. It is terminology that conveys a sense of looking at your established relationship as the substantial part and another person as an enhancer, appendage, etc. She becomes an object in that perspective.
It would be different if you said, "I'd like a relationship with a woman and if she is open to being with my partner, and there is a mutual attraction there, we'd love that. But we don't expect it." When you say, "we want to add a third," it doesn't sound like the person is as important as what her role should be - THE THIRD. So, people call you out on that. And if any women who would be open to in that kind of arrangement come across your post, they think, "Ugh, not another unicorn hunter who wants me there just to make them both equally happy." If I were a bi woman open to being with a couple, and I read your sentence saying "Another woman will be able to satiate his prowess..." I'd roll my eyes and groan because that sounds like you just want a sex toy. I think that anyone who wants a respectful relationship, where the aim is to connect emotionally or on a heart level and be seen as an individual, would steer clear of you. So, think about what it is you really want and the impression you're making in asking for it.
For more insight: Added to, Joining In