Further to my previous post, here is the upside.
There are lots of ways to be together that are not monogamy. You can be polyamorous, non-monogamous, in-an-open-relationship, swingers, Ethical Sluts, or [insert your own word here]. Each has as many different meanings as there are people who practise it, so for the sake of simplicity I will use 'poly' and try to cover as many aspects of all forms of non-monogamy as possible
Polyamory is not polygamy.
Polygamy sounds like it's closely related to monogamy, and linguistically it is. Within religious denominations such as the Mormons, to quote an American example, it's also an unequal system, in which men get to have multiple wives while women are expected to stay faithful to their shared husband. It's another male-focussed social system with sightly different ratios of male:female. (I'm aware this is not true of every polygamous society worldwide, but once again I'm focussing on traditions descended from the Abrahamic laws). Polyamory is about both women and men having freedom to have sex with more than one person. It also holds none of the expectations on women of sexual availability, child-bearing and economic dependency that are frequently found within polygamous Mormonism.
Polyamory is not a way of cheating without saying you're cheating.
Poly is, like good kink, safe, consensual, and well-negotiated. Poly doesn't just happen to a relationship without mutual consent. It needs to be talked about, common ground is sought and found, and boundaries are made clear. Poly couples have ground rules, and partners need to stick to those. Some people insist on only same-sex partners, on only playing when out of the family home, on only playing as a couple, on meeting a partner's potential partners first.
Which brings me to...
Poly is safe if you make it safe.
A major ground rule for most poly couples is safer sex. While monogamy pretty much removes the opportunity to be frank about the use on contraception and barrier methods with other partners, poly negotiations pretty much require an agreement on safer sex.
That is where fidelity to your primary/ies happens - you don't just trust them with your heart, but quite literally with your life. And they trust you with theirs - and if that isn't a big damn incentive not to take risks that you might take if it was just you, I don't know what is.
Poly does not make you a bad parent
I'll be brief on this one, not having tried it myself. But it's still asumed that poly parents, like gay parents, means children automatically suffer from growing up in an atypical household. There are always going to be some parents who don't put their child first, but why should anyone assume they're always the non-monogamous, non-straight, non-Normal ones? This is not the case. Having extra adults involved in their care is a good thing for children, they get more attention and affection and time. The nuclear family, that recent invention, can stand to grow and stretch and include more people in more bonds. Extended families of multiple adults were caring for children long before economic forces created the two-parent, one-earner-one-carer model - which, by the way, is going out the window anyway, now that women can work and one wage won't feed a growing family. Extended familys of parents and partners who are happily together in whatever combination are a bonus for a child.
Poly is not just for bisexuals...
...although the two do go together like Baileys and coffee. But poly can work for every orientation, and the notion that it's just us bisexuals, having all the cake and eating it as usual, is an unfounded stereotype. You can be straight and be poly, and you can be gay and poly. People of any orientation can have a great time with partner-swaps, swinging, V-shaped relationships, cruising together and getting involved in group sex for any number of players. But, speaking personally, being bi and poly is a particular delight. Being bi makes it tough to be monogamous, because not only do all the girls look hotter once you're paired up, but all the boys do too. It's good to be in a relationship which doesn't preclude you from taking an active interest in the other end of your attraction spectrum.
I'd argue the same is true for people of all sexual orientations - one person, however much they make you swoon, however well they know you, is unlikely to be the only person in the world you're turned on to. It happens, certainly, but there is much more extra-curricular attraction going on than we think, and it's not always guilty. You can be twenty years happily married and still nourish a yen for James Taylor, or spinning with first love but also watching every Kiera Knightly film you can get your hands on. And that's OK. Poly makes it negotiatedly all right, not just to have those attractions, but to act on them.
Poly keeps you talking.
Lovers have to talk. It's the difference between foundering and salvation. Having an atypical relationship means you take nothing for granted and discuss everything - you have to renogiate everything with your partner because the rules aren't the same. This is also true of kinky relationships, queer relationships of every sort, and any sort of relationship that swims against the tide of normality. You have stuff you have to talk about, because you're in unmapped terrain, and all the practise comes in handy when the trivial little deal-breakers like the dishes and the dusting rear their ugly heads.
Poly is frubbly.
There's a word I want to see written into the OED, not because it sounds nice, but because we need it. It's the diametric opposite of jealousy. The nearest antonym of jealousy I can find is 'trusting', which is defining jealousy as irrational posessiveness. It's more than that. It's biting insecurity and silent fear and frustration and stress. Frubble is the opposite to all those things. Frubble is when you send your Significant Other off to be with zir Other Significant Other, knowing they'll both be glad to see you when you all meet tomorrow. It's when you phone your primary to talk about the amazing weekend you had with someone else, and ze's genuinely pleased for you. It's when you are glad that your lover is finding something ze needs with another lover, something that you didn't necesarily have - be it a shared interest in cooking, complementary kinks, or an eostrogen-based body.
Poly is fun
Poly is tricky. In one way, it's monogamy squared, cubed, endlessly expanded to include new people. It's harder to make time, to balance commitments, to keep things even and open and negotiated. But it can also let you out to have fun and give you a place to come back to. It can allow friends to bond sexually and erotic interest to stay above-board and safe. It lets men, women and everyone else do things they've been told, for no good reason but Normality, that they can't.
I'm not saying it's not tough. It's hard work, it can go horrendously wrong, it can be painful and difficult and break your heart. It can also be fantastic, uplifting, comforting, sexy and so uniquely good it makes you cry.
Bit like love, really.