Got linked to this thread from @JaneQSmythe's post in this one, I hope I'm not breaking any thread necromancy rules by answering here after the last post is nearly two years old...
I'm very much anti-marriage, just like Olderwoman. I really don't see the benefit of keeping around an institution that not only muddles the lines between religion and secular law, but also has its roots in ownership and sexism (basically, father gives away his daughter (his = he made her from his semen, he fed her with the fruits of his handwork, so she's his to decide over), who hopefully is still virginal lest her market value gets ruined, to her new owner, whom she is now to serve as breedingstock for his, hopefully forever unpolluted by other men's semen, bloodline... ugh!). That, to me, is just about the exact opposite of what poly is about... so yes, I tend to generally see them as incompatible ideas at their core. Poly is a profoundly modern, equalist, and democratic concept (all of which deeply appeal to me), while marriage to me is a fossilized relic from patriarchal and hierarchical days that we've made good progress to overcome in today's Western industrial societies.
On a much more personal level, as I'm asexual myself, there's the whole sexual consummation aspect. If my marriage could at any time get annulled at the drop of a hat, without even needing to go through a full divorce process, because something I know I will never do with my spouse is legally seen as the prime criterion of what makes it a "real" marriage... then why bother with signing a contract that can be voided at a moment's notice? Seems pointless to me.
In all honesty, I'd be all for abandoning the concept of secular marriage sooner rather than later (religion can keep it if they want, but there should be no secular legal benefits whatsoever involved; I see no reason for marriage to be treated differently than, say, getting baptized); I do, however, support the concept of civil unions/"registered households", who should have those legal and economic benefits. (The fact that in areas where both marriage and c.u. exist side by side under current law, the latter tend to be massively inferior in terms of benefits is scandalous.)
I'd also would definitely hope for massive improvements of flexibility in legislation as to who can get civil unions - far beyond just the matter of straight/gay orientation, there should also be equal access to c.u. regardless of number of people involved and type of relationship between them (poly or mono, sexual or not, romantic or not, blood related or not)... while I don't give a damn about whether people who don't conform to "one cismale, one cisfemale, strictly mono till death does them part" can marry (actually, seeing as what its roots are, barring people who can't or won't breed with each other, and only with each other, from marriage makes perfect sense!). I'm fine letting religion alone decide over a concept that really only has relevance to traditional religion anymore - and if that means no gay marriage, ever, then so be it, I don't see the fuss. *shrugs*
When only the hardcore religious get married at all anymore, and a civil union between a brother, a sister, their Auntie Bertha, and Bob who may or may not sleep with any one of those three no longer gets a single eyebrow raised, then I'll be happy. (It'll be a long time coming, if ever, I know... but one can dream. )
Btw, you noticed I left love completely out of the picture? No coincidence, because marriage isn't about love. I consider it was a bad, bad idea that romance and marriage ever got muddled (a concept younger than people think; marrying for "flimsy reasons" such as love would have been seen as laughably immature by most people just two or three centuries ago - you married for money and politics, not for emotions; if you also came to be friends or *gasp!* even romantic lovers with your spouse - well, lucky you, but that's a bonus, not a necessity to make the marital contract work as intended), and consider arranged marriages to be truer to the concept... both are equally outdated.