When people say OPP it's usually THEIR reaction to the idea of being with only one man. They frame is as derogatory and "man-induced" because they wouldn't accept a situation like that themselves. What they seem to forget is that, more often than not, it's a JOINT negotiation.
Take a fictional couple who are monogamous (and let's say married so we know that they've promised monogamy to each other) and throughout the course of the marriage, the wife has come to realise that she's bisexual and would like the opportunity to explore that side of her. Now, she might go to her husband and say "I think I'm bisexual, or at the least I'm curious in being with a woman, but I love you loads and I wouldn't want to do anything without your blessing." Now, if this same woman came onto these forums, she'd have her detractors shouting "ONE PENIS POLICY!" when really she's got exactly what she wanted.
Same sort of thing with unicorns - there's a lot of couples out there who would genuinely be interested in a three-way relationship, but because this doesn't match up to other people's experiences of things, they're derided for being hunters etc etc.
Now, I will say that there are people out there who "deserve" the criticism - I'm talking about the kind of person who hasn't really examined themselves, hasn't really communicated with their partners and just thinks "I don't want another guy sticking it to my wife" or "A partner that loves us both equally would be great" without any real desire there. However, there's also people out there who have communicated and have worked on their various issues and built a relationship which they want, and STILL want something like a OPP or a triad relationship.
Using my own relationship as an example, we're essentially in a completely open relationship - the only rules are practise safe sex and no close friends/family members of other partners. Those two rules are simply to protect our bodies (stds etc) and our lives (since not everyone knows/would approve etc). Now, despite the fact that are rules are very few, we both would still like to form a triad relationship. She's bisexual, likes threesomes and has always imagined a relationship with three people. Who is anyone on this forum to deride her preferences by coming out with a snarky "unicorn-hunter" comment?
A lot of people are very quick to jump on the "OPP", for example a post in another topic said:
I wonder what the limits are he imposed on you? OPP?
The problem with that is that it INSTANTLY portrays the woman as the victim and the man as a controlling decision maker. If there's one thing I've learnt from poly relationships it's that limits are MUTUALLY AGREED. There is no way I would ever be able to get my primary to agree to a OPP unless it is EXACTLY WHAT SHE WANTED AS WELL. I cannot IMPOSE a limit, since the other person can simply not agree to follow it.
I guess what I'm saying is that there's still a lot of "my poly is better than your poly" going on, where people react based on THEIR situation rather than the situation of the person posting. If we follow that logic, then any time I see anyone with any rule that isn't "be safe and don't out me" I should tell them that they're doing it wrong, and that my polyamory is far superior for our lack of rules. Except that's not how it works, because everyone wants something different, and I don't think it's too much to ask that people show enough respect for each others' choices, even if they wouldn't make the same choices themselves.
Just to note, this isn't a specific accusation at any one person (although I understand that by quoting someone it could appear that way). The quote was intended as an example rather than as an attack. My point is that, as a community, we need to keep this a safe space where people can come and be free of judgement. There's enough judgement out in the world simply for BEING poly that this should be the place where we don't judge each other for the decisions we make WITHIN poly. As long as someone can say that they've made an informed decision with their partner(s) then tbh they should be free to do as they please.