The benefits thing would be bound to be really, really complex, along with lots of other legal issues, but isn't life already pretty complex to begin with? I find it funny that church and state never really had anything to do with marriage until early modernity, when they jumped in and claimed authority over it (which church has finally been getting elbowed out of). Polyamory works for some people, and there's no legal red tape around that - though I imagine the people that want the legal recognition or sanctification for it, in terms of polygamy, have their reasons. Islam (and some kinds of African Christianity) makes an interesting test case. The stated rule there is that a man can only take on more than one wife is he is able and sworn to protect them all equally (and yes, this only works in one direction). No doubt this is very rarely the case. A secular legal apparatus around that might stand a better chance of ensuring that kind of protection.