Here are a few thoughts – because, contrary to many people's assumptions this isn't a situation I stumbled into in some weakened addled state fleeing a terrible marriage. I worked hard on myself in order to leave that situation, and after leaving to make sure I had worked through all the stuff I needed to.
Apologies if this is long, 
but I want to address a few recurring themes, directly and clearly.
1 – The if you're looking to engineer a setup in which abuse of power, coercion and fostering of dependency to the detriment of the most vulnerable you couldn't do much better than a modern stereotypical monogamous marriage. In generations gone by marriages usually existed with a context of a close knit social and familial community, where people were held accountable to others who were intimately involved in their everyday lives – extended families, close knit villages etc. These days so many marriages exist in large part isolation – we live away from our families, in our little nuclear bubbles, and often nobody really knows what goes on. The perfect conditions for abusers to do their worst, almost invisible to the outside world.
2 – In any kind of setup where more than two people are involved you automatically have other people who hold you accountable for your behaviour within your relationships. I'm I'm a shit to DP, OSO will call me on it. If I'm unfair to OSO, DP will call me on it. If either of them are unkind to me they will be called on it. If they are unkind to each other, I will call them on it. We sit down, the three of us, every month over afternoon tea and sometimes a long walk and we look at what's working and not. If someone needs something they're not getting we talk about it and see if we can find a better way of doing things. If someone is finding something difficult, we talk about it. It's not for everyone, I know, but for us it's a very healthy, open and deliberate way of going about constructing a relationship structure which works for all the people in it.
3 – People seem to assume that everyone wants a 24/7 relationship with a single person who meets all their emotional, physical, intellectual needs. There's nothing wrong with that if it's what works for you. But it's never been something that I wanted – I lived alone, happily for five years before I got married, and loved it. I had deep reservations about sharing my space with another person 24/7, but went along with what was 'expected' – against my better judgement (and lived to regret it). DP and OSO have had an open relationship almost from the get go. They never felt the need or desire to just be the two of them, and while that's taken a number of forms over the years, from swinging to the relationship the three of us are in now, they've always taken the view that we are enriched by the people in our lives, and there was no need, for them, to draw some kind of line around the form that can take, between friendship and sexuality, for example, or to restrict the intimacy that other relationships can have. Monogamy works for a lot of people, but it's not for me, and it's never been for DP and OSO.
4 – 'It will never GO anywhere' is a recurring theme. Like there's some ultimate destination for relationship and a single track to get there. (The relationship 'escalator' is often how it's referred to in books on non-standard relationships.) Thing is, I'm not trying to 'get' anywhere other than to seek to be the best version of myself that I can be for the people I love. I have as much time with DP and OSO as I want or need. I thoroughly enjoy the time I spend with them individually and together. I have time on my own, which I need and enjoy. I have wonderful dedicated time with my children where my focus is 100% on them. I spend time with my friends. I have a fulfilling career, financial independence and material and emotional security. Where or what exactly should I be aiming for? If I need more than I'm getting in my current relationship I can seek to adjust the way my current relationship operates, or I can seek another relationship to add to it – and we all have the agreed freedom to do that. But at the moment, the only thing I wish to have beyond what I currently do is the ability to have all the people I love free to spend time together if they choose to. (And I'm working hard to create the circumstances where that can happen.) The destination people have in mind seems to be a mono-normative nuclear arrangement, and quite simply, and it's not somewhere I'm interested in going.
5 – The other pervasive idea is that I just haven't met the right man, and if/when I did I would want all that. But I'm in my forties. I've had several of those kinds of relationships over the years (largely because I didn't think there was an alternative) with some amazing people, some less than amazing people. And simply put, I don't think there's a man (or woman) in the world who could make a that relationship model fit me. It just doesn't.
Equally I'm not saying that poly is 'better' than monogamy or that it's for everyone or even most people. Some people like and want and need that single person, always around, to the exclusion of all others. Most, perhaps. But some of us just don't like, want or need that.
I'm not being 'short changed'. I don't 'deserve better'. I have the kind of relationship I want with people who I enjoy spending time with in many different ways. I'm happy, they're happy. It's only people outside of the relationship who don't know us who aren't. Those who spend time with the three of us, namely close friends and extended family – even those who didn't understand it or perhaps approve initially – can see we're happy together and good for each other.
I spoke to OSO on Thursday night - the three of us were out together learning about cocktails and having dinner – about this thread. She's not interested in contributing directly – she's not an online person – but having made sure that I wasn't being unduly upset by some of the things being said, thought I should continue to answer questions as they arise. Because the biggest challenge we, and others like us, face, is the judgements and hostility of people who don't understand the way we've chosen to live our lives but think they do, and have a right to pass judgement and make all sorts of assumptions about it on that basis. And to talk about it openly, to answer questions and challenges as they are put to me, might in some small way contribute to changing that. Eventually. So that, for example, a woman might not fear losing custody of her children by the simple fact of being involved in a poly relationship.
She assures you all that she's quite happy, and finds it amusing that so many of you would be happier with our set up if she and I were fucking each other too, but to say sorry, she's tried that in the past, and discovered that she's 100% straight.
Sorry, this was REALLY long. 