For most poly people out there, there's really nothing wrong with monogamy. Both mono and poly are valid relationship models, and everyone chooses the one which best fits them. I really wish I could wholeheartedly agree with this... however, I must admit that monogamy clashes with my ethics. And here's why:
It makes a person feel guilty about having a feeling that I consider to be one of the most important feelings of all. Our ability to connect with other people, to admire other people for their greatness, to be attracted to what we value (spiritually and physically), the ability to love – is probably our most vital characteristic in this life. Monogamy takes a somewhat controversial stance towards it – this property is considered wonderful in a person if he is single, but once he already has one romantic relationship, this very same property is labeled as extremely immoral and wrong. Well, newsflash, everyone: the ability to fall in love does not miraculously disappear when one finds a partner! OK, so maybe for some people it does. Maybe. For most of us, it does not; and polyamory finds this wonderful. In monogamy, however, it is usually considered very wrong to even be attracted to someone else – even if you never act upon these feelings. So, basically, if you're in a monogamous relationship, and there's this other person that you realize is absolutely awesome, then you should feel somewhat guilty. You should reject this realization. (Especially if it accompanies a realization that you love the way her eyes look when she smiles. Boy, if you even think that, you're in trouble, and you should be very ashamed of yourself. Monogamy usually asserts that it is OK to communicate with people of the opposite sex as long as you don't notice that they have a body at all, eyes included.)
And this is what ticks me off. For me, it's all very simple, really: Love is great. Sex is great. If a doctrine states that love and sex between two consenting adults is immoral, then that doctrine is wrong.
Note: some mono folks will say that in their eyes it is OK to be attracted to other people, as long as you don't have sex with them. Usually, however, there's a great deal of hypocrisy involved – the same people who state this as their belief get extremely pissed off if their partner actually admits to having such feelings.
I must stress that when I say "monogamy is wrong", I am nevertheless extremely tolerant to it. (OK, this world is monogamous, so I'd be in big trouble if I weren't tolerant…) I feel about it in about the same way as a vegetarian feels about eating murdered animals – he believes that it is wrong and people shouldn't do it, but he doesn't actually stop talking to all people who eat meat. Moreover, he doesn't start preaching about it at every meal; but he will also not try hard to avoid the topic. This belief is an important part of him, and he doesn't feel like he should hide it. This is more or less exactly how I feel about polyamory.